Modern history




IN THE FIRST FEW DAYS AFTER THE PUMP HANDLE’S REMOVAL, an engineer by the name of Edmund Cooper began examining the Broad Street epidemic on behalf of the Metropolitan Commission of Sewers. Rumors that sewer excavations had unearthed the decaying but still pestilent corpses from the plague burial grounds had been buzzing through the neighborhood. Even the newspapers had implicated the old pesthouse fields. (The Daily News had published a letter on September 7 accusing the sewer builders of unearthing an “immense quantity of human bones” during their excavations in the area.) With these scandalous accusations floating about, the Commission dispatched Cooper to investigate the claim. Cooper quickly arrived at the conclusion that the bodies of two-hundred-year-old plague victims posed little threat to the neighborhood, whether they had been disturbed by sewer construction or not. It was clear from the Weekly Returns—and from Cooper’s on-scene investigating—that the sewer construction had not likely played a role, given the geographic dispersal of deaths. But Cooper needed a way to represent these patterns in an intelligible manner that both the laypeople of the neighborhood and his supervisors might understand. So he created a map of the outbreak. He modified an existing plan of the neighborhood that showed the new sewer lines, adding visual codes to indicate both the location of cholera deaths and the site of the original plague pit. For each house that had suffered a loss, Cooper drew a black bar by the address, followed by a succession of thin lines indicating how many deaths had occurred at that address. In the northwest corner of the map, roughly centered over Little Marlborough Street, Cooper drew a circle inscribed with the words “Supposed Location of Plague Pit.” A quick glance at the map made it clear that the outbreak had been triggered by some other source: the deaths were concentrated several blocks to the southeast of the ancient burial site. Only a handful of deaths had occurred within Cooper’s circle, and the houses to the immediate south and east of the circle had been spared entirely. If some noxious effluvium had risen out of the plague pit, surely the residents living directly on top of the pit would have suffered the worst casualty rate.

Cooper’s original layout would be copied and expanded in another map produced for the Board of Health investigation that included data from the more extensive survey that had been carried out that fall. Once again, the map exonerated the plague pit, though the committee ultimately included the sewer lines as a potential source of miasmatic poisoning in the area. Both maps were well-crafted specimens of the new art of dot mapping—that is, representing the spatial path of an epidemic by marking each case with dots (or bars) on a map. They were both attempts to tell the story of the Broad Street outbreak from the bird’s-eye view, to see the patterns of the disease as it erupted through the neighborhood. They were both superbly detailed: old and new sewer lines were documented with distinct markings; each gulley hole was represented by an icon on the map, along with ventilators and side entrances and the street number of every house in the parish. Even the neighborhood pumps were included. But as exacting as Cooper’s map was, it ultimately had too much detail to make sense of the story. The connection between the Broad Street pump and the surrounding deaths was lost under the sheer mass of data that Cooper had charted. For a map to explain the true cause behind the Broad Street outbreak, it needed to show less, not more.

JOHN SNOW BEGAN WORKING ON HIS FIRST MAP OF THE Broad Street outbreak sometime in the early fall of 1854. Its initial form, which he shared publicly at a meeting of the Epidemiological Society in December, resembled Cooper’s survey, with two small modifications: Each death was represented by a thick black bar, which made the houses that had suffered significant deaths more vivid on the map. And the detail on the map was reduced, with everything but the basic street layout and the icons representing the thirteen public water pumps that served the greater Soho area eliminated. The visual impact of the map was striking. Because it represented a larger section of London—from Hanover Square in the west to Soho Square in the east, and all the way south to Piccadilly Circus—eleven of the pumps were shown to be entirely clear of local cholera cases. The Little Marlborough Street pump had a few black bars in its immediate vicinity, but they were nothing compared with the concentration of death around the Broad Street pump, black bars lining the nearby streets like solemn high-rises. Without a prominent icon for the Broad Street pump, the other dot maps of the epidemic had presented an amorphous shape, like a cloud hanging over western Soho. But when you emphasized the pumps in the image, the map took on a new clarity. Cholera wasn’t lingering over the neighborhood in a diffuse form. It was radiating out from a single point.

In effect, Snow had given the death and darkness of the Broad Street outbreak a new kind of clarity. His first map has been rightly celebrated for its persuasiveness, and variations of it have been reproduced in countless textbooks on cartography, information design, and public health. A landmark 1911 textbook on epidemiology, Sedgwick’s Principles of Sanitary Science and Public Health, included a dozen pages on the Broad Street case and featured a revised version of the map prominently. Thanks to that continued attention, the map has become the defining symbol of the entire Broad Street outbreak. But its significance has been somewhat misunderstood. The black bars marking the ghosts of Soho were a striking visual element, but they were hardly Snow’s invention. Not only had dot maps been created to visualize previous cholera outbreaks, but at least one (Cooper’s) had already been created to document the Broad Street outbreak itself before Snow began work on his map. Part of what made Snow’s map groundbreaking was the fact that it wedded state-of-the-art information design to a scientifically valid theory of cholera’s transmission. It was not the mapmaking technique that mattered; it was the underlying science that the map revealed.

Snow modified his original map for publication in two places—the Vestry Committee’s report and the second edition of his own monograph on cholera. Augmented by the new data on the outbreak that Whitehead and others had assembled, the second version of the map contained Snow’s most significant contribution to the field of disease mapping. (Ironically, it goes unmentioned in Edward Tufte’s extensive account of Snow’s mapmaking in Visual Explanations, which almost single-handedly elevated Snow’s work to the information-design canon.) After presenting to the Epidemiological Society, Snow had realized that his original map was still vulnerable to a miasmatic interpretation. Perhaps the concentration of deaths around the Broad Street pump was merely evidence that the pump was releasing noxious fumes into the air. And so Snow realized he needed a way to represent graphically the foot-traffic activity around the pump that he had so painstakingly reconstructed. He needed to show lives, not just deaths; he needed to show the way the neighborhood was actually traversed by its residents.

To solve this problem, Snow drew upon a centuries-old mathematical tool that would later be termed the Voronoi diagram. (It is unlikely that Snow knew anything of the device’s history, though he was certainly the first to apply it to disease mapping.) A Voronoi diagram conventionally takes the shape of a two-dimensional field made up of points surrounded by “cells.” The cells define the region around each point that is closer to that particular point than any other point in the diagram. Imagine a football field with a point on each goal line. The Voronoi diagram of that field would be divided into two cells, the demarcation between them being at the fifty-yard line. If you stand anywhere on the field on the home-team side of the fifty-yard line, you are closer to the point on the home team’s goal line than you are to the point on the other goal line. Most Voronoi diagrams, of course, involve many points scattered about in unexpected ways, resulting in a honeycomb pattern of cells surrounding their local points.

What Snow set out to do with his second map was to create a Voronoi diagram using the thirteen pumps as points. He would diagram a cell that showed the exact subsection of addresses on the map that were closer to the Broad Street pump than they were to any other pump. But these distances would have to be calculated on foot-traffic terms, not the abstract distances of Euclidean geometry. The cell was warped by the erratic arrangement of streets in Soho. Some addresses were closer to Broad Street as the crow flies, but if you actually paced the routes out by foot, winding your way through the crooked alleys and side streets of Soho, another pump turned out to be closer. It was, as the historian Tom Koch astutely notes, a map organized as much around time as around space: instead of measuring the exact distance between two points, it measured how long it took to walk from one point to another.

And so the second version of the map—the one that made it into both Snow’s monograph and the Vestry report—included a slightly odd, wandering line that circumscribed the center of the outbreak, roughly in the shape of a square with five or six areas jutting out, like small peninsulas, into the surrounding neighborhood. This was the area encompassing all those residents for whom the quickest trip for water was to the Broad Street pump. Superimposed over the black bars that marked each death, the amorphous shape took on sudden clarity: each peninsula extended out to embrace another distinct cluster of deaths. Beyond the circumference of the cell, the black bars quickly disappeared. Snow’s visual case for his waterborne theory revolved around a striking correspondence between two shapes: the shape of the outbreak area itself, and the shape of best proximity to the Broad Street pump. If the cholera were somehow spreading as a miasmatic emission from the pump, the shape of the neighborhood deaths would have looked quite different: not a perfect circle, perhaps, since some houses would have been more vulnerable than others. But it certainly wouldn’t have followed so precisely the contours of street-level (i.e., foot-traffic) proximity to the Broad Street well. The miasma wouldn’t be influenced by the eccentricities of street layouts, after all, and it certainly wouldn’t be influenced by the location of other neighborhood pumps.

And so the ghosts of the Broad Street outbreak were reassembled for one final portrait, reincarnated as black bars lining the streets of their devastated neighborhood. In dying, they had collectively made a pattern that itself pointed to a fundamental truth, though it took a trained hand to make that pattern visible. And yet, however elegant its design, the map’s immediate influence was far less dramatic than folklore has it. The map didn’t solve the mystery of the outbreak. It didn’t lead to the pump handle’s removal and thus bring an end to the epidemic. In fact, it failed to sell the Board of Health on the merits of the waterborne theory. Yet despite those reservations, Snow’s map deserves its iconic status. The case for the map’s importance rests on two primary branches: its originality and its influence.

The originality of the map did not revolve around the decision to map an epidemic, or even the decision to encode deaths in bars etched across the street diagram. If there was a formal innovation, it was that wobbly circumference that framed the outbreak in the second version, the Voronoi diagram. But the real innovation lay in the data that generated that diagram, and in the investigation that compiled the data in the first place. Snow’s Broad Street map was a bird’s-eye view, but it was drawn from true street-level knowledge. The data that it sketched out in graphic form was a direct reflection of the ordinary lives of the ordinary people who made up the neighborhood. Any engineer could have crafted a dot map from William Farr’s Weekly Returns. But the Snow map drew on a deeper, more intimate, source: two Soho residents talking to their neighbors, walking the streets together, sharing information about their daily routines, and tracking down the long-departed émigrés. Neighborhood demographics had been projected onto maps before, of course, but invariably the projections involved the official interventions of the census takers or the Board of Health. Snow’s map—with Whitehead’s local knowledge animating it—was something else entirely: a neighborhood representing itself, turning its own patterns into a deeper truth by plotting them on a map. The map is a brilliant work of information design and epidemiology, no doubt. But it is also an emblem of a certain kind of community—the densely intertwined lives of a metropolitan neighborhood—an emblem that, paradoxically, was made possible by a savage attack on that community.

As for influence, it’s pretty to think of John Snow unveiling the map before the Epidemiological Society to amazed and thunderous applause, and to glowing reviews in The Lancet the next week. But that’s not how it happened. Its persuasiveness seems obvious to us now, living as we do outside the constraints of the miasma paradigm. But when it first began circulating in late 1854 and early 1855, its impact was far from dramatic. Snow himself seems to have thought that his South London Water Works study would ultimately be the centerpiece of his argument, the Broad Street map merely a piece of supporting evidence, a sideshow.

The tide of scientific opinion would eventually turn in Snow’s favor, and when it turned, the Broad Street map grew in stature. Most accounts of the outbreak reproduced the map in some fashion—so often, in fact, that copies of copies began appearing in textbooks, described erroneously as original reproductions. (Most of them lacked the critical Voronoi diagrams.) As the waterborne theory of cholera became increasingly accepted, the map was regularly invoked as a shorthand explanation of the science behind the theory. It was easier to point to those black bars emanating ominously from the pump than it was to explain the whole idea of microorganisms invisible to the human eye. The map may not have had the impact on its immediate audience that Snow would have liked, but something about it reverberated in the culture. Like the cholera itself, it had a certain quality that made people inclined to reproduce it, and through that reproduction, the map spread the waterborne theory more broadly. In the long run, the map was a triumph of marketing as much as empirical science. It helped a good idea find a wide audience.

SNOW’S MAP MAY HAVE HAD A CRUCIAL SHORT-TERM IMPACT as well, though this is closer to an inference than an empirical fact. We know that Henry Whitehead’s interest in the waterborne theory turned decisively after Snow gave him a copy of his revised cholera monograph in the late winter of 1855. That monograph contained the second edition of Snow’s map. It’s entirely possible that seeing all those deaths radiating out from the Broad Street pump played a role in changing the curate’s mind. He had spent more time than anyone working through the intimate details of those lives and deaths—first attending the sick as a clergyman, then investigating the outbreak as an amateur detective. It must have been a revelation to see all that data rendered from above for the first time.

Persuading an assistant curate of the merits of the waterborne theory might seem like a minor accomplishment. But Whitehead’s investigations in 1855 were ultimately as decisive as Snow’s in solving the Broad Street mystery. His “conversion experience” reading Snow’s monograph set him off in search of the index case, eventually leading him to baby Lewis. The discovery of baby Lewis led to York’s excavation of the pump, which confirmed a direct connection between the pump and the cesspool at 40 Broad.

It’s conjecture, of course, but it’s nonetheless entirely reasonable to assume that without the Reverend Whitehead’s contributions the Vestry Committee would have never blamed the outbreak on the Broad Street pump. Without an index case and an unequivocal link to the well water, without the support of one of the neighborhood’s most beloved characters, it would have been so much easier for the Vestry Committee to equivocate, to blame the outbreak on the neighborhood’s generally pitiful sanitary standards—in the streets and in the houses, in the water and in the air. It would have been so much easier for the Vestry Committee to fall back on the miasmatic haze of the Board of Health report. But the final compilation of evidence had been too overwhelming for such stock explanations. When you combined Snow’s original data with Whitehead’s more exhaustive investigation, when you factored in the index case and the decaying brickwork, the conclusion was inevitable: the pump was the source of the outbreak.

The Vestry Committee’s verdict meant that for the first time an official committee investigation had endorsed the waterborne theory. It was a small victory, since the vestry had no power over public-health issues outside Soho, but it gave Snow and his future allies something that Snow had long sought: an official endorsement. In the years and decades that followed, the Vestry Committee report grew in influence as the story of the Broad Street outbreak was re-told. Slowly, over time, it occluded the Board of Health investigation altogether. The twelve pages devoted to Broad Street in Sedgwick’s Principles of Sanitary Science and Public Health quote extensively from the Vestry report, while the Board of Health verdict goes unmentioned. The vast majority of the retellings of the Broad Street case fail to mention the signal fact that among the public-health authorities of the day, Snow’s investigation was of no significance.

Rewinding the tape of history and imagining alternative scenarios is always a fanciful exercise, but it can be instructive. If the Vestry Committee had not endorsed the waterborne theory, then the Broad Street episode would likely have entered the historical record as yet another example of miasma’s deadly reach: a crowded, unsanitary neighborhood suffused with hideous smells that got its comeuppance. Snow’s interventions would have remained the work of an illustrious maverick, an outsider with an unproven theory who failed to convince anyone other than a panicked Board of Governors that had removed a pump handle out of desperation. No doubt science would ultimately have come around to the waterborne theory, but it might well have taken decades longer without the clarity and reproducibility of the Broad Street story and its accompanying map. How many thousands more might have died in that interval?

It is a subtle chain of causal connections, but a plausible one nonetheless. The map helps tip Whitehead toward the waterborne theory, which prods him to discover the index case, which necessitates the second excavation, which ultimately tips the Vestry Committee toward Snow’s original theory. And the endorsement of the Vestry Committee rescues Broad Street from the side of the miasmatists. It becomes the most powerful and seductive proxy for Snow’s waterborne theory, thus accelerating the adoption of the theory by the very same public-health institutions that had renounced it so thoroughly at the time of the outbreak. The map may not have persuaded Benjamin Hall of the dangers of contaminated water in the spring of 1855. But that doesn’t mean it didn’t change the world in the long run.

Imagining the chain of events this way makes one fact overwhelmingly clear: John Snow may have been instrumental in first identifying the pump as the likely culprit behind the outbreak, but Whitehead ultimately supplied the crucial evidence for establishingthe pump’s role. The shorthand version of the Broad Street case invariably settles on the image of the visionary scientist, working alone against the dominant paradigm, discovering the secret cause behind a terrible plague. (Whitehead is often mentioned in popular accounts, but usually as a sort of dutiful apprentice, helping Snow with the door-to-door surveys.) But Broad Street should be understood not just as the triumph of rogue science, but also, and just as important, as the triumph of a certain mode of engaged amateurism. Snow himself was a kind of amateur. He had no institutional role where cholera was concerned; his interest in the disease was closer to a hobby than a true vocation. But Whitehead was an amateur par excellence. He had no medical training, no background in public health. His only credentials for solving the mystery behind London’s most devastating outbreak of disease were his open and probing mind and his intimate knowledge of the community. His religious values had brought him into close contact with the working poor of Soho, but they had not blinded him to the enlightenments of science. If part of the significance of Snow’s second map lay in the way it empowered the community to represent itself, Whitehead was the conduit that made that representation possible. Whitehead was not an expert, an official, an authority. He was a local. That was his great strength.

And here lies an antidote of sorts to the horror of Broad Street, to the grisly image of entire families dying together in their single-room flats: the image of Snow and Whitehead building an unlikely friendship in those late winter months of 1855, drawn together by a terrible outbreak of disease in their neighborhood and, ironically, by Whitehead’s initial skepticism about Snow’s theory. We know very little about the personal interaction between the two men, beyond the crucial data they exchanged, beyond Snow’s sharing of his monograph, and his prophetic statements about the future of cholera. But it is clear from Whitehead’s subsequent recollections that a powerful bond formed between them—the quiet, awkward anesthesiologist and the compulsively social curate—a bond forged both by living through an urban battleground of unimaginable terror, but also by jointly unearthing the secret cause behind the carnage.

This is not mere sentimentality. The triumph of twentieth-century metropolitan life is, in a real sense, the triumph of one image over the other: the dark ritual of deadly epidemics replaced by the convivial exchanges of strangers from different backgrounds sharing ideas on the sidewalk. When John Snow first stepped up to the Broad Street pump in early September 1854, it was by no means clear which image would be victorious. London seemed to be destroying itself. You could leave town for a weekend and come back to find ten percent of your neighbors being wheeled down the street in death carts. That was life in the big city.

Snow and Whitehead played a small but defining role in reversing that trend. They solved a local mystery that led, ultimately, to a series of global solutions—solutions that transformed metropolitan living into a sustainable practice and turned it away from the collective death drive that it threatened to become. And it was precisely their metropolitan connection that made this solution possible: two strangers of different backgrounds, joined by circumstance and proximity, sharing valuable information and expertise in the public space of the great city. The Broad Street case was certainly a triumph of epidemiology, and scientific reasoning, and information design. But it was also a triumph of urbanism.

John Snow would never get to experience that triumph in its entirety. In the first few years that followed the outbreak, supporters of the waterborne theory grew in number and in visibility. Snow’s monograph had included both the Broad Street case and the South London water-supply study, and the combination seemed to produce converts at a much greater clip than the original monograph had six years before. John Sutherland, prominent inspector for the Board of Health, made several public statements that offered at least a partial endorsement of the waterborne theory. William Farr’s Weekly Returns grew increasingly supportive of the theory. Several publications appeared that argued for the waterborne theory without crediting Snow for the original insight—including a few that credited William Budd with the discovery of cholera’s waterborne nature. Perhaps aware that his legacy would ultimately revolve around his cholera investigations, Snow responded to these papers with politic, but firm, letters to the medical journals, reminding his colleagues of his precedence in these matters.

Still, miasma retained its hold over many, and Snow himself was often subjected to derisive treatment by the scientific establishment. In 1855, he gave his testimony in Parliament on behalf of the “offensive trades” before a committee on the Nuisances Removal Act. Snow argued eloquently that infectious diseases were not spread through the foul smells emitted by the bone-boilers and gut spinners and tanners of industrial London. Again, he drew upon reasoned statistical analysis, arguing that the laborers who worked in these establishments would have had a much greater incidence of disease than the general public if the miasma were somehow breeding epidemics. The fact that they did not show a disproportionate rate of contagion—despite their immersion in the fumes—meant that the cause of disease lay elsewhere.

Benjamin Hall, ever the miasmatist, expressed open disbelief at Snow’s testimony. Edwin Chadwick would shortly after denounce Snow’s reasoning as illogical. But the real assault would come in an unsigned editorial in The Lancet that tore into Snow with remarkable fury and disdain:

Why is it, then, that Dr. Snow is so singular in his opinion? Has he any facts to show in proof? No!… But Dr. Snow claims to have discovered that the law of propagation of cholera is the drinking of the sewage-water. His theory, of course, displaces all other theories. Other theories attribute great efficacy in the spread of cholera to bad drainage and atmospheric impurities. Therefore, says Dr. Snow, gases from animal and vegetable decomposition are innocuous! If this logic does not satisfy reason, it satisfies a theory; and we all know that theory is often more despotic than reason. The fact is, that the well whence Dr. Snow draws all sanitary truth is the main sewer. His specus, or den, is a drain. In riding his hobby very hard, he has fallen down through a gully-hole and has never since been able to get out again.

The confidence of the miasmatists couldn’t last forever. In June 1858, a relentless early-summer heat wave produced a stench of epic proportions along the banks of the polluted Thames. The press quickly dubbed it the “Great Stink”: “Whoso once inhales the stink can never forget it,” the City Press observed, “and can count himself lucky if he live to remember it.” Its overwhelming odors shut down Parliament. As the Times reported on June 18:

What a pity… that the thermometer fell ten degrees yesterday. Parliament was all but compelled to legislate upon the great London nuisance by the force of sheer stench. The intense heat had driven our legislators from those portions of their buildings which overlook the river. A few members, bent upon investigating the matter to its very depth, ventured into the library, but they were instantaneously driven to retreat, each man with a handkerchief to his nose.

But a funny thing happened when William Farr calculated his weekly returns for those early weeks of June. The rates of death from epidemic disease proved to be entirely normal. Somehow the most notorious cloud of miasmatic air in the history of London had failed to produce even the slightest uptick in disease mortality. If all smell was disease, as Edwin Chadwick had so boldly pronounced more than a decade before, then the Great Stink should have conjured up an outbreak on the scale of 1848 or 1854. Yet nothing out of the ordinary had happened.

It’s easy to imagine John Snow taking great delight in the puzzling data from the Weekly Returns, perhaps writing up a brief survey for The Lancet or the London Medical Gazette. But he never got the opportunity. He had suffered a stroke in his office on June 10, while revising his monograph on chloroform, and died six days later, just as the Great Stink was reaching its peak above the foul waters of the Thames. He was forty-five years old. More than a few friends wondered if his many experiments inhaling experimental anesethetics in his home lab had brought on his sudden demise.

Ten days later, The Lancet ran this brief, understated item in its obituary section:

DR. JOHN SNOW—This well-known physician died at noon on the 16th instant, at his house in Sackville-street, from an attack of apoplexy. His researches on chloroform and other anaesthetics were appreciated by the profession.

Snow might have hoped that cholera would prove central to his legacy, but in the first obituary that ran after his death it didn’t even warrant a mention.

AFTER YEARS OF BUREAUCRATIC WAFFLING, THE GREAT STINK finally motivated the authorities to deal with the crucial issue that John Snow had identified a decade before: the contamination of the Thames water from sewer lines emptying directly into the river. The plans had been in the works for years, but the public outcry over the Great Stink had tipped the balance. With the help of the visionary engineer Joseph Bazalgette, the city embarked on one of the most ambitious engineering projects of the nineteenth century: a system of sewer lines that would carry both waste and surface water to the east, away from Central London. The construction of the new sewers was every bit as epic and enduring as the building of the Brooklyn Bridge or the Eiffel Tower. Its grandeur lies belowground, out of sight, and so it is not invoked as regularly as other, more iconic, achievements of the age. But Bazalgette’s sewers were a turning point nonetheless: they demonstrated that a city could respond to a profound citywide environmental and health crisis with a massive public-works project that genuinely solved the problem it set out to address. If Snow and Whitehead’s Broad Street investigation showed that urban intelligence could come to understand a massive health crisis, Bazalgette’s sewers proved that you could actually do something about it.

North of the Thames, the plan for the new sewers involved three main lines, each at different levels of elevation, running eastward parallel to the river. On the south side, there were to be two main lines. All the city’s existing surface water and waste lines would empty into one of these “intercepting” sewers, and the contents would then flow—and in some cases be pumped—several miles east of the city. On the north side, they drained into the Thames at Barking; on the south, the outfalls were located at Crossness. The sewers only discharged into the Thames during high tide, after which the seaward pull of low tide would flush the city’s waste out to the ocean.

It was a demonically complicated undertaking, given that the city already had a complicated infrastructure of pipes and rail stations and buildings—not to mention a population of nearly three million people—that Bazalgette somehow had to work around. “It was certainly a very troublesome job,” he would later write, with typical English understatement. “We would sometimes spend weeks in drawing out plans and then suddenly come across some railway or canal that upset everything, and we had to begin all over again.” Yet somehow, the most advanced and elaborate sewage system in the entire world was largely operational by 1865. The numbers behind the project were staggering. In those six years, Bazalgette and his team had constructed eighty-two miles of sewers, using over 300 million bricks and nearly a million cubic yards of concrete. The main intercepting sewers had cost only £4 million to construct, which would be roughly $250 million today. (Of course, Bazalgette’s labor costs were much cheaper than today’s.) It remains the backbone of London’s waste-management system to this day. Tourists may marvel at Big Ben or the London Tower, but beneath their feet lies the most impressive engineering wonder of all.

The best way to appreciate the scale of Bazalgette’s achievement in person is to stroll along the Victoria or Chelsea embankments on the north side of the river, or along the Albert Embankment on the southern shore. Those broad, attractive esplanades were built to house the massive low-elevation interception lines that ran parallel to the Thames. Beneath the feet of those happy riverside pedestrians enjoying the view and the open air, beneath the cars hurtling along north of the river, there lies a crucial, hidden boundary, the last line of defense that keeps the city’s waste from reaching the city’s water supply.

That low-elevation northern sewer was one of the final lines to be completed, and the delays in building it turned out to play a determining role in London’s last great outbreak of cholera. In late June 1866, a husband and wife living in Bromley-by-Bow in East London fell ill with cholera and died a few days later. Within a week a terrible outbreak of cholera erupted in the East End—the worst the city had seen since the ravages of 1853–1854. By the end of August, more than four thousand people had died. This time it was William Farr who did the first round of detective work. Puzzled by the sudden explosion of cholera in the city after a decade of relative dormancy, Farr thought of his old sparring partner, John Snow, and his surveys of the South London water companies that had brought Snow so regularly to the Registrar-General’s Office. Farr decided to break down these new deaths along water-supply lines, and when he did, the pattern was unmistakable. The great majority of the dead had been customers of the East London Water Company. This time around, Farr wouldn’t waste time with miasmatic objections. He didn’t know how the East London supply had been contaminated, but clearly there was something deadly in that water. To waste time would be to condemn untold thousands to their deaths. Farr immediately ordered that notices be posted in the area advising residents not to drink “any water which has not been previously boiled.”

Still, mysteries remained. Bazalgette’s sewers were supposed to have cut off the fatal feedback loop between London’s outputs and inputs, its waste and its water supply. And the East London Water Company claimed to use extensive filtering at all of its reservoirs. If some contaminant had somehow made its way out of the city’s sewers, it should have been picked up by the East London filters before being passed on to the wider population. Farr sent a letter to Bazalgette, who immediately wrote back, apologetically, to say that the drainage system in that part of the city had not been activated yet. “It is unfortunately just the locality where our main drainage works are not complete,” he explained. The low-level sewer had been constructed, but Bazalgette’s contractors had yet to finish the pumping station required to elevate the sewage so that gravity could continue to pull it down toward its ultimate outfall at Barking. And so the intercepting line in that area was not in use yet.

Attention then turned to the East London Water Company. Initially, company representatives swore that all their water had been run through state-of-the-art filter beds at their new covered reservoirs. But reports had surfaced of some customers discovering live eels in their drinking water, which suggested that the filters were not perhaps working optimally. An epidemiologist named John Netten Radcliffe had been assigned to investigate the outbreak, and he began looking into the filtering system in place at East London. Only a few months before, Radcliffe had read a memoir of the Broad Street outbreak authored by a curate who had played some role in the investigation. In the absence of John Snow, it occurred to Radcliffe that this individual might bring some valuable experience to this latest epidemic. And so the amateur epidemiologist Henry Whitehead was brought back to help solve one last case of poisoned water.

Radcliffe and Whitehead, along with other investigators, quickly uncovered a number of negligent practices at the East London company that had allowed the nearby River Lea to contaminate the groundwater around the company’s reservoir at Old Ford. Eventually, the index cases at Bromley-by-Bow were tracked down; the doomed couple’s water closet turned out to empty into the River Lea less than a mile from the Old Ford reservoir. In the end, the link to the East London water supply proved to be even more statistically pronounced than the link to the Broad Street pump had been in 1854. Ninety-three percent of the dead were eventually found to be East London Water customers.

This time, the verdict was nearly unanimous, and Snow’s visionary research was widely acknowledged. Farr himself delivered some of the most powerful words in testimony before Parliament the year after the outbreak. He began in a satiric mode, deriding the commercial interests that sustained the miasma theory despite so much evidence to the contrary:

As the air of London is not supplied like water to its inhabitants by companies, the air has had the worst of it both before Parliamentary Committees and Royal Commissions. For air no scientific witnesses have been retained, no learned counsel has pleaded; so the atmosphere has been freely charged with the propagation and the illicit diffusion of plagues of all kinds; while Father Thames, deservedly reverenced through the ages, and the water gods of London, have been loudly proclaimed immaculate and innocent.

Of course, one man had in fact served as “learned counsel” for the atmosphere, in much reviled testimony ten years before. And in turn, Farr acknowledged John Snow’s defining role:

Dr. Snow’s theory turned the current in the direction of water, and tended to divert attention from the atmospheric doctrine.… The theory of the East wind with cholera on its wings, assailing the East End of London, is not at all borne out by experience of previous epidemics.… An indifferent person would have breathed the air without any apprehension; but only a very robust scientific witness would have dared to drink a glass of the waters of the Lea at Old Ford after filtration.

Farr’s conversion to Snow’s doctrine was so complete that he literally rewrote history to make it appear as though Snow’s ideas had more initial success than they had actually enjoyed. In the introduction to his report on the 1866 outbreak, Farr, alluding to the investigation into the Broad Street case, delivers this stunning account of the Board of Health committee’s findings:

The final report of the scientific committee proved conclusively the extensive influence of water as a medium for the diffusion of the disease in its fatal forms.… Dr. Snow’s view that the cholera-stuff was distributed in all its activity through water was confirmed. The special report… inculpated the Broad-street pump to some extent in the terrible outbreak of the St. James district. But the subject was further and more conclusively investigated by a committee, aided by Dr. Snow and by the Rev. Henry Whitehead.

Either Farr was willfully distorting the record, or—like so many subsequent accounts—his memory of the Vestry Committee’s investigation had suppressed the Board of Health report. Recall the exact wording of the Board of Health’s “confirmation” of Snow’s theory: “After careful inquiry, we see no reason to adopt this belief. We do not find it established that the water was contaminated in the manner alleged.” With confirmations like that, who needs criticisms?

Still, the waterborne hypothesis had at long last entered the dominant scientific paradigm. It pleased Whitehead to know that he had once again helped his old friend’s ideas find a larger audience. Even The Lancet came around, editorializing in the weeks after the 1866 outbreak:

The researches of Dr. Snow are among the most fruitful in modern medicine. He traced the history of cholera. We owe to him chiefly the severe induction by which the influence of the poisoning of water-supplies was proved. No greater service could be rendered to humanity than this; it has enabled us to meet and combat the disease, where alone it is to be vanquished, in its sources or channels of propagation.… Dr. Snow was a great public benefactor, and the benefits which he conferred must be fresh in the minds of all.

Apparently Dr. Snow found a way out of that “gully-hole” after all.

BY THE LAST DECADES OF THE NINETEENTH CENTURY, THE germ theory of disease was everywhere ascendant, and the miasmatists had been replaced by a new generation of microbe hunters charting the invisible realm of bacterial and viral life. Shortly after discovering the tuberculosis bacillus, the German scientist Robert Koch isolated Vibrio cholerae while working in Egypt in 1883. Koch had inadvertently replicated Pacini’s discovery of thirty years earlier, but the Italian’s work had been ignored by the scientific establishment, and so it was Koch who won the initial round of acclaim for identifying the agent that had caused so much trauma over the preceding century. History would come around to the Italian, though. In 1965, Vibrio cholerae was formally renamedVibrio cholerae Pacini 1854.

Even these advances were not enough to convince a few remaining stalwarts—like Edwin Chadwick, who went to his grave in 1890 an unrepentant believer in the disease-causing powers of miasma. But most public-health institutions reoriented themselves around the new science. Establishing sanitary water supplies and waste-removal systems became the central infrastructure project of every industrialized city on the planet. The appearance of the electrical grid, around the turn of the century, tends to attract more attention, but it was the building of the invisible grid of sewer lines and freshwater pipes that made the modern city safe for the endless consumer delights that electricity would bring. Bazalgette’s project was a model for the world to emulate. By 1868, the pumping station at Abbey Mills was finally completed, which meant the northern branch of Bazalgette’s grand sewer system was fully operational. By the mid-1870s, the entire system was online. Sewage continued to be pumped into the eastern end of the Thames until 1887, when the city began dumping waste into the open sea.

The changes ushered in by the sewer system were manifold: fish returned to the Thames; the stench abated; the drinking water became markedly more appetizing. But one change stood out above all the others. In all the years that have passed since Henry Whitehead helped track down the Old Ford reservoir contamination in 1866, London has not experienced a single outbreak of cholera. The battle between metropolis and microbe was over, and the metropolis had won.

Cholera would continue to terrorize Western cities into the first decades of the twentieth century, but with London’s successful engineering project as a model, the outbreaks usually prodded the local authorities into modernizing their civic infrastructure. One such outbreak hit Chicago in 1885, after a heavy storm flushed the sewage collecting in the Chicago River far enough into Lake Michigan that it reached the intake system for the city’s drinking water. Ten percent of the city’s population died in the ensuing outbreak of cholera and typhoid, and the deaths ultimately led to the city’s epic effort to reverse the flow of the Chicago River, sending the sewage away from the water supply. Hamburg had built a modern sewage system in the 1870s, modeled largely on London’s, but the design had been flawed, and in 1892 cholera returned to claim nearly ten thousand lives out of a population one-seventh that of London. Because the major cholera epidemics of the preceding sixty years had all jumped the English Channel from Hamburg, Londoners waited anxiously as news of the German outbreak came over the wires. But their concern was unwarranted. Bazalgette’s defenses held, and the cholera never appeared on British shores.

By the 1930s, cholera had been reduced to an anomaly in the world’s industrialized cities. The great killer of the nineteenth-century metropolis had been tamed by a combination of science, medicine, and engineering. In the developing world, however, the disease continues to be a serious threat. A strain of V. cholerae known as “El Tor” killed thousands in India and Bangladesh in the 1960s and 1970s. An outbreak in South America in the early 1990s infected more than a million people, killing at least ten thousand. In the summer of 2003, damage to the water-supply system from the Iraq War triggered an outbreak of cholera in Basra.

There is a fearful symmetry to these trends. In many ways, the struggles of the developing world mirror the issues that confronted London in 1854. The megacities of the developing world are wrestling with the same problems of uncharted and potentially unsustainable growth that London faced 150 years ago. In 2015, the five largest cities on the planet will be Tokyo, Mumbai, Dhaka, São Paulo, and Delhi—all of them with populations above 20 million. The great preponderance of that growth will be driven by so-called squatter or shantytown developments—entire sprawling cities developed on illegally occupied land, without any traditional infrastructure or civic planning supporting their growth. The scavenger classes of Victorian London have been reborn in the developing world, and their numbers are truly staggering. There are a billion squatters on earth now, and some estimates suggest that their numbers will double in the next twenty years. It’s entirely possible that a quarter of humanity will be squatters by 2030. All the characters of the Victorian underground economy—the mud-larks and toshers and costermongers—may have largely disappeared from cities in the developed world, but everywhere else on the planet their numbers are exploding.

Squatter cities lack most of the infrastructure and creature comforts of developed metropolitan life, but they are nonetheless spaces of dynamic economic innovation and creativity. Some of the oldest shantytown developments—the Rocinha area in Rio de Janeiro, Squatter Colony in Mumbai—have already matured into fully functioning urban areas with most of the comforts we’ve come to expect in the developed world: improvised wood shacks giving way to steel and concrete; electricity; running water; even cable television. The main road in the squatter village of Sultaneyli in Istanbul is lined with six-story buildings, bustling with the commerce of ordinary city life: banks, restaurants, shops. And all of this has been accomplished without title deeds, without urban planners, without government-created civic infrastructure, on land that it is, technically speaking, illegally occupied. The squatter communities are not, by any measure, sinkholes of poverty and crime. They are where the developing world goes to get out of poverty. The writer Robert Neuwirth puts it best in his mesmerizing account of squatter culture, Shadow Cities: “With makeshift materials, they are building a future in a society that has always viewed them as people without a future. In this very concrete way, they are asserting their own being.”

But that hope needs to be tempered with caution. The squatters still face significant obstacles. Arguably the most pressing obstacle is the one that confronted London a century and a half ago: the lack of clean water. Over 1.1 billion people lack access to safe drinking water; nearly 3 billion—almost half the planet—do not possess basic sanitation services: toilets, sewers. Each year 2 million children die from diseases—including cholera—that result directly from these unsanitary conditions. And so the megacities of the twenty-first century will have to learn all over again the lessons that London muddled through in the nineteenth. They’ll be dealing with 20 million people, instead of 2 million, but the scientific and technological wisdom available to them far exceeds what Farr and Chadwick and Bazalgette had at their disposal.

Some of the most ingenious solutions now being proposed take us back to the waste-recycling visions that captivated so many Victorian minds. The inventor Dean Kamen has developed two affiliated machines—each the size of a dishwasher—that together can provide electricity and clean water to rural villages or shantytown communities that lack both. The power generator runs off a readily available fuel—cow dung—though Kamen says it will run off “anything that burns.” Its output can power up to seventy energy-efficient bulbs. The ambient heat from the generator can be used to run the water purifier, which Kamen nicknamed Slingshot. The device accepts any form of water, including raw sewage, and extracts the clean water through vaporization. Kamen’s prototype includes a “manual” featuring a single instruction: just add water. Just as the pure-finders once roamed London, recycling dog excrement for the leather tanners, the squatters of tomorrow may end up solving the sanitation problems of their community by using the very substances—animal and human waste—that cause the problems in the first place.

One cannot be unduly optimistic about how these megacities will face their potential crises in the coming years. There may be new technologies that enable the squatter communities to concoct public health solutions on their own, but governments will obviously need to play a role as well. It took industrial London a hundred years to mature into a city with clean water and reliable sanitation. The scavenger classes that Mayhew analyzed with such detail no longer exist in London, but even the wealthiest cities in the developed world continue to face problems of homelessness and poverty, particularly in the United States. But the developed cities no longer appear to be on a collision course with themselves, the way London did in the nineteenth century. And so it may take the megacities of the developing world a century to reach that same sense of equilibrium, and during that period there will no doubt be episodes of large-scale human tragedy, including cholera outbreaks that will claim far more lives than were lost in Snow’s time. But the long-term prospects for urban life, even in these vast new sprawling “organisms,” are solid ones. It’s likely the megacities will mature faster than London did, precisely because of all the forms of expertise that were in embryo during the Broad Street events: epidemiology, public infrastructure engineering, waste management and recycling. And of course all that expertise has been greatly amplified by the connective powers of the Web, linking institutional knowledge with the local knowledge of amateurs to an extent that Snow and Whitehead could never have imagined.

It has never been easier for that local knowledge to find its way onto a map, making patterns of health and sickness (as well as less perilous matters) visible to experts and laypeople in new ways. The descendants of Snow’s Broad Street map are now ubiquitous on the World Wide Web. Instead of Snow and Whitehead knocking on doors, and William Farr tabulating physicians’ reports, we now have far-flung networks of health providers and government officials reporting outbreaks to centralized databases, where they are automatically mapped and published online. A service called GeoSentinel tracks infectious diseases among travelers; the CDC publish a weekly update on the current state of influenza in the United States, along with a near-endless array of charts and maps documenting the different strains of flu circulating through the national bloodstream. The popular ProMED-mail e-mail list offers a daily update on all the known disease outbreaks flaring up around the world, which surely makes it the most terrifying news source known to man. The technology has advanced dramatically, but the underlying philosophy remains the same: that there is something profoundly enlightening about seeing these patterns of life and death laid out in cartographic form. The bird’s-eye view remains as essential as it was back in 1854. When the next great epidemic does come, maps will be as crucial as vaccines in our fight against the disease. But again, the scale of the observation will have broadened considerably: from a neighborhood to an entire planet.

The influence of the Broad Street maps extends beyond the realm of disease. The Web is teeming with new forms of amateur cartography, thanks to services like Google Earth and Yahoo! Maps. Where Snow inscribed the location of pumps and cholera fatalities over the street grid, today’s mapmakers record a different kind of data: good public schools, Chinese takeout places, playgrounds, gay-friendly bars, open houses. All the local knowledge that so often remains trapped in the minds of neighborhood residents can now be translated into map form and shared with the rest of the world. As in 1854, the amateurs are producing the most interesting work, precisely because they have the most textured, granular experience of their community. Anyone can create a map that shows you where streets intersect and where hotels are; we’ve had maps like that for centuries. The maps now appearing are of a different breed altogether: maps of local knowledge created by actual locals. They’re street-smart. They map the intangibles: blocks that aren’t safe after dark, playgrounds that could use a renovation, local restaurants that have room for strollers, overvalued real estate offerings.

Even ordinary Web pages can be explored geographically now. Both Yahoo! and Google have established a standard convention for “tagging” a given piece of information—a blog post, say, or a promotional website—with geographic coordinates that are automatically interpreted by search engines. Someone writes into an online community forum with a complaint about a local park and tags the message with the park’s exact location; someone writes up a mini-review of a new restaurant; someone posts a notice about a summer sublet that they’re offering. Up to now, all of those individual pieces of data possessed a location in the information space of the Web, in that they were associated with a URL—a “uniform resource locator.” Now those items can possess a location in real-world space as well. In the near future, we’ll use these geo-tags as we explore a new city, in much the way that we use search engines to explore the space of the Web today. Instead of looking for Web pages associated with a keyword or phrase, we’ll look for pages associated with the streetcorner we’re standing on. We’ll be able to build instantly the kind of bird’s-eye view of a neighborhood that Snow and Whitehead stitched together by hand over months of investigation.

These are technologies that thrive in urban centers, because they grow more valuable the more densely populated the environment is. A suburban cul-de-sac is unlikely to have a significant number of Web pages associated with it. But a streetcorner in a big city might well have a hundred interesting links: personal stories, reviews about the hot new bar around the corner, a potential date who lives three blocks away, a hidden gem of a bookstore—perhaps even a warning about a contaminated water fountain. These digital maps are tools for making new kinds of sidewalk connections, which is why they are likely to be less useful in communities without sidewalk culture. The bigger the city, the more likely it is that you’ll be able to make an interesting link, because the overall supply of social groups and watering holes and local knowledge is so vast.

Jane Jacobs observed many years ago that one of the paradoxical effects of metropolitan life is that huge cities create environments where small niches can flourish. A store selling nothing but buttons most likely won’t be able to find a market in a town of 50,000 people, but in New York City, there’s an entire button-store district. Subcultures thrive in big cities for this reason as well: if you have idiosyncratic tastes, you’re much more likely to find someone who shares those tastes in a city of 9 million. As Jane Jacobs wrote:

Towns and suburbs… are natural homes for huge supermarkets and for little else in the way of groceries, for standard movie houses or drive-ins and for little else in the way of theater. There are simply not enough people to support further variety, although there may be people (too few of them) who would draw upon it were it there. Cities, however, are the natural homes of supermarkets and standard movie houses plus delicatessens, Viennese bakeries, foreign groceries, art movies, and so on, all of which can be found co-existing, the standard with the strange, the large with the small. Wherever lively and popular parts of cities are found, the small much outnumber the large.

The irony, of course, is that digital networks were supposed to make cities less attractive, not more. The power of telecommuting and instant connectivity was going to make the idea of densely packed urban cores as obsolete as the walled castle-cities of the Middle Ages. Why would people crowd themselves into harsh, overpopulated environments when they could just as easily work from their homestead on the range? But as it turns out, many people actually like the density of urban environments, precisely because they offer the diversity of Viennese bakeries and art movies. As technology increases our ability to find these niche interests, that kind of density is only going to become increasingly attractive. These amateur maps offer a kind of antidote to the scale and complexity and intimidation of the big city. They make everyone feel like a native, precisely because they draw upon the collective wisdom of the real natives.

City governments are exploring these new mapping technologies as well. Several years ago New York City rolled out its pioneering 311 service, which may well be the most radical enhancement of urban information management since William Farr’s Weekly Returns. Modeled after the on-demand tech-support lines that New York mayor Michael Bloomberg built into the computer terminals that made him rich, as well as on a few smaller programs in cities such as Baltimore, 311 is ultimately three distinct services wrapped into one. First, it is a kinder, gentler version of 911; in other words, 311 is the number New Yorkers call when there’s a homeless person sleeping near the playground—and not the number they call when someone’s breaking into their apartment. (During the first year of 311’s operation, the total number of 911 calls decreased for the first time in the city’s history.) The 311 service also functions as a kind of information concierge for the city, offering on-demand information about all the city’s services. Citizens can call to find out if the concert in Central Park has been canceled due to rain, if alternate-side parking is in effect, or the location of the nearest methadone clinic.

But the radical idea behind the service is that the information transfer is genuinely two-way. The government learns as much about the city as the 311 callers do. You can think of 311 as a kind of massively distributed extension of the city’s perceptual systems, harnessing millions of ordinary “eyes on the street” to detect emerging problems or report unmet needs. (Bloomberg himself is notorious for calling in to report potholes.) During the blackout of 2003, many diabetic New Yorkers grew increasingly apprehensive about the shelf life of room-temperature insulin. (Insulin is traditionally kept refrigerated.) The city’s emergency planners hadn’t anticipated those worries, but within a matter of hours, Bloomberg was addressing the topic in one of the many press conferences broadcast over the radio that night. (Insulin, it turns out, remains stable for weeks at room temperature.) The insulin issue had trickled up the city’s command chain thanks to calls into the 311 line. Those diabetics dialing in during the blackout got an answer to their question, but the city got something valuable in return: the calls had made them aware of a potential health issue that hadn’t occurred to them before the lights went out.

Already 311 is having an impact on the city government’s priorities. In the first year of operation, noise issues dominated the list of complaints: construction sites, late-night parties, bars and clubs spilling out onto sidewalks. The Bloomberg administration has subsequently launched a majority quality-of-life initiative combating city noise. Much as the COMPSTAT system revolutionized the way the police department fought crime by mapping problem areas with new precision, the 311 service automatically records the location of each incoming complaint in a massive Siebel Systems call-center database that feeds information throughout the city government. Geo-mapping software displays which streets have chronic pothole troubles and which blocks are battling graffiti.

Increase the knowledge that the government has of its constituents’ problems, and increase the constituents’ knowledge of the solutions offered for those problems, and you have a recipe for civic health that goes far beyond the superficial appeal of “quality of life” campaigns. When people talk about network technology revolutionizing politics, it’s usually in the context of national campaigns: Internet fund-raising, or political blogging. But the most profound impact may be closer to home: keeping a neighborhood safe and clean and quiet, connecting city dwellers to the immense array of programs offered by their government, creating a sense that individuals can contribute to their community’s overall health, just by dialing three numbers on a phone.

All of these extraordinary new tools are descendants of the Broad Street investigation and its maps. The great promise of urban density is that it thrusts so many diverse forms of intelligence, amateur and professional, into such a small space. The great challenge is figuring out a way to extract all that information and spread it throughout the community. The information that Snow and Whitehead sought revolved around the terror and senselessness of a deadly outbreak, but their approach has survived to tackle a vast array of problems, now augmented by modern information technology. Some of these problems are equally life-threatening (“When will my insulin go bad?”), but most of them involve the small concerns of everyday life. Add up enough of those small concerns, though, and you get a genuine transformation in your lived environment, not to mention a renewed sense of civic participation, a sense that your street-level understanding of your neighborhood can make a difference in the larger scheme of things. When Snow and Whitehead took their local knowledge of the Soho community and transformed it into a bird’s-eye view of the outbreak, they were helping to invent a way of thinking about urban space whose possibilities we are still exploring today. It was an act with profound implications for the medical establishment, of course, but it was something else as well: a model for managing and sharing information that has implications that extend far beyond epidemiology.

The model involves two key principles, both of which are central to the way cities generate and transmit good ideas. First; the importance of amateurs and unofficial “local experts.” Despite Snow’s advanced medical training, the Broad Street case might well have been ultimately ruled in favor of miasma had it not been for the untrained local expertise of Henry Whitehead. Cities are invariably shaped by their master planners and their public officials; Chadwick and Farr had a tremendous impact on Victorian London—most of it positive, despite the miasma diversions. But in the last instance, the energy and vitality and innovation of cities comes from the Henry Whiteheads—the connectors and entrepreneurs and public characters who make the urban engine work at the street level. The beauty of technologies like 311 is that they amplify the voices of these local experts, and in doing so they make it easier for the authorities to learn from them.

The second principle is the lateral, cross-disciplinary flow of ideas. The public spaces and coffeehouses of classic urban centers are not organized into strict zones of expertise and interest, the way most universities or corporations are. They’re places where various professions intermingle, where different people swap stories and ideas and skills along the way. Snow himself was a kind of one-man coffeehouse: one of the primary reasons he was able to cut through the fog of miasma was his multidisciplinary approach, as a practicing physician, mapmaker, inventor, chemist, demographer, and medical detective. But even with that polymath background, he still needed to draw upon an entirely different set of skills—more social than intellectual—in the form of Henry Whitehead’s local knowledge.

WHEN SNOW PROPHESIED TO HIS FRIEND THAT THE TWO OF them might not live to see the waterborne theory vindicated, he was only half right. Snow died before his ideas could change the world, but Whitehead lived another four decades, long enough to see London fend off the Hamburg outbreak of 1892. Whitehead remained at St. Luke’s until 1857, and then for the next seventeen years was a vicar at various parishes around the city, spending much of his time working on the problem of juvenile delinquency. In 1874, he left the city to serve on a series of ministries in northern England. Shortly before he left, his fellow investigator from the East End outbreak of 1866, John Netten Radcliffe, wrote of Whitehead’s role in the Broad Street case:

In the Broad Street outbreak of cholera not only did Mr. Whitehead faithfully discharge the duties of a parish priest, but by a subsequent inquiry, unique in character and extending over four months… he laid the first solid groundwork of the doctrine that cholera may be propagated through the medium of drinking water.… This doctrine, now fully accepted in medicine, was originally advanced by the late Dr. Snow; but to Mr. Whitehead unquestionably belongs the honour of having first shown with anything approaching to conclusiveness the high degree of probability attaching to it.

Henry Whitehead died in 1896, at the age of seventy. Until death, a portrait of his old friend John Snow hung in his study—to remind him, as Whitehead put it, “that in any profession the highest order of work is achieved, not by fussy empirical demands for ‘something to be done,’ but by patient study of the eternal laws.”

How much would Henry Whitehead recognize were he to stroll down the streets of Soho today? The visible signs of the Broad Street outbreak would be long gone. Indeed, it is the peculiar nature of epidemic disease to create terrible urban carnage and leave almost no trace in the infrastructure of the city. The other great catastrophes that afflict cities—fires, earthquakes, hurricanes, bombs—almost invariably inflict vast architectural damage alongside the human body count. In fact, that’s how they tend to do their killing: by destroying human shelter. Plagues are more insidious. The microbes don’t care about buildings, because the buildings don’t help them reproduce. So the buildings get to continue standing. It’s the bodies that fall.

The buildings have changed nonetheless. Almost every structure that stood on Broad Street in the late summer of 1854 has been replaced by something new—thanks in part to the Luftwaffe, and in part to the creative destruction of booming urban real estate markets. (Even the street names have been altered. Broad Street was renamed Broadwick in 1936.) The pump, of course, is long gone, though a replica with a small plaque stands several blocks from the original site on Broad Street. A block east of where the pump once stood is a sleek glass office building designed by Richard Rogers with exposed piping painted a bold orange; its glassed-in lobby hosts a sleek, perennially crowded sushi restaurant. St. Luke’s Church, demolished in 1936, has been replaced by the sixties development Kemp House, whose fourteen stories house a mixed-use blend of offices, flats, and shops. The entrance to the workhouse on Poland Street is now a quotidian urban parking garage, though the workhouse structure is still intact, and visible from Dufours Place, lingering behind the postwar blandness of Broadwick Street like some grand Victorian fossil.

But there is much that Whitehead would recognize in the streets of Soho today, even though the buildings have been replaced and the rents have risen. The coffee shops are now mostly national chains, but the rest of the neighborhood is thick with the small-scale energy of local entrepreneurs. The mineral-teeth manufacturers have given way to video production facilities, hipster music stores with vinyl records in the window, Web design firms, boutique ad agencies, and “Cool Britannia” bistros—not to mention the occasional sex worker, a reminder of Soho’s sordid days in the seventies. Everywhere the neighborhood is thriving with the passions and provocations of dense metropolitan living. The streets feel alive, precisely because they are animated by the intersecting paths of so many separate human lives. That there is safety and energy and possibility in those intersections—and not a looming fear of death—is part of the legacy of the battle fought on those streets 150 years ago. Perhaps it is even the most important part.

On Broad Street itself, only one business has remained constant over the century and a half that separates us from those terrible days in September 1854. You can still buy a pint of beer at the pub on the corner of Cambridge Street, not fifteen steps from the site of the pump that once nearly destroyed the neighborhood. Only the name of the pub has changed. It is now called The John Snow.



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