The Ghost Map

by Steven Johnson

  Snow knew that his case would also revolve around the inverse situation: residents who lived near the pump who survived, because, for one reason or another, they had opted not to drink from the poisoned well. He reviewed Farr’s list again, looking this time for telltale absences. There were a handful of deaths reported at 50 Poland Street. On its own, this was a predictable number: Poland Street lay immediately to the north of the pump, well within Snow’s imagined border. But in scanning the list, Snow realized that the number was strikingly low, because 50 Poland Street was the address of the St. James Workhouse, home to 535 people. Two deaths was routine for a household of ten living off of Broad Street. A population of five hundred living close to the pump should have seen dozens of death. As Whitehead had already learned from his daily rounds, the workhouse—despite its destitute and morally suspect inmates—had been something of a sanctuary from the outbreak. When Snow interrogated the workhouse directors, an explanation immediately jumped out at him: the workhouse had a private supply from the Grand Junction Water Works, which Snow knew from his earlier research to be one of the more reliable sources of piped water. The workhouse also had its own well on the premises. They had no reason to venture out to the Broad Street pump for water, even though it lay not fifty yards from their front door.

  Snow noticed another telling absence on Farr’s list. With seventy workers, the Lion Brewery at 50 Broad was the second-largest employer in the immediate vicinity. Yet not a single death was recorded for that address in Farr’s list. It was possible, of course, that the workers had gone home to die, and so Snow paid a visit to the Lion’s proprietors, Edward and John Huggins, who reported with some bafflement that the plague had passed over their establishment. Two workers had reported mild cases of diarrhea, but not a single one had shown severe symptoms. When Snow inquired about the water supply on the premises, the Hugginses replied that, like the workhouse, the brewery had both a private pipeline and a well. But, they explained for the benefit of the teetotaling doctor, they rarely saw their men drink water at all. Their daily rations of malt liquor usually satisfied their thirst.

  Later, Snow would visit the Eley Brothers factory, where he found the situation much more dire. The proprietors reported that dozens of their employees had fallen ill, many of them dying in their own homes over the first few days of the epidemic. When Snow noticed the two large tubs of water that the brothers kept on premises for their employees to drink from, he scarcely needed to ask where the water had originated.

  Snow had heard through the grapevine that the Eley brothers’ mother and their cousin had recently perished of cholera as well, both of them far removed from Golden Square. The coincidence must have immediately struck Snow; perhaps he even thought back to the experimentum crucis gauntlet thrown down by the London Medical Gazette so many years before. No doubt, considering Snow’s discretion, he posed the question delicately: Had Susannah Eley by any chance consumed some of the water from the Broad Street pump? It must have been an anguished moment for Snow: how to extract the information he needed without revealing that the brothers’ thoughtfulness had been the agent of their mother’s demise. Snow’s taciturn demeanor would have helped him as the brothers described their regular deliveries of pump water to Hampstead; a more volatile investigator might have responded to the revelation of that crucial clue with more emotion. But whatever emotion he showed the Eley brothers, when he stepped out of the factory into the bright light of Broad Street, he must have thought to himself with some satisfaction that the case was coming together quite nicely indeed. The miasmatists might finally have met their match.

  THERE IS A KIND OF MYTHOLOGY THAT STORIES LIKE THIS one tend inevitably to drift toward: the lone genius shaking off the chains of conventional wisdom through the sheer force of his intellect. But in explaining Snow’s battle against the miasma theory and the medical establishment, it’s not sufficient to point to his brilliance or his tenacity alone, though no doubt those characteristics played a crucial role. If the dominance of the miasma model was itself shaped by multiple intersecting forces, so, too, was Snow’s ability to see it for the illusion that it was. Miasma was the intellectual equivalent of a contagious disease; it had spread through the intelligentsia with an alarming infection rate. So why was John Snow immune?

  Part of the answer lies in Snow’s study of ether and chloroform. The underlying insight that brought him his first round of acclaim was that the vapors of ether and chloroform had remarkably predictable effects on human beings. If you controlled the density of the gas, there was very little variation in the way humans—not to mention the frogs and birds in Snow’s lab—would respond to inhalation. Without that predictability, of course, Snow would have never been able to build a thriving career for himself as an anesthesiologist; the risks and unreliability of the procedure would have greatly outweighed the benefits. Ether was itself a poisonous vapor—a kind of miasma in its own right—and yet it seemed entirely indifferent to the “inner constitution” of the humans who inhaled it. If ether had followed the pattern described by some of the miasmatists, it would have triggered radically different responses, depending on the inner constitution of each patient—perhaps causing some to become preternaturally alert, while inducing laughter in others, and rendering others senseless in seconds. But Snow had watched thousands of patients be sedated by the gas over the preceding six years, and he knew firsthand how mechanistic the process was. His whole career was, in a sense, a testimony to the predictable physiological effects of inhaled vapors. And so, when the miasma theorists invoked the inner constitution to explain why half the population of a room might succumb to poisonous vapors while the other half emerged unscathed, Snow was naturally inclined to view the theory with some suspicion.

  His experience with choloform and ether had also endowed Snow with an intuitive grasp of the way that gases disperse in the environment. Ether could be deadly in a concentrated form, delivered directly to the patient’s lungs. But a doctor delivering it, standing a foot away from the patient, wouldn’t feel its effects in the slightest, because the density of ether molecules in the air dropped at a precipitous rate the farther removed one was from the inhaler itself. This principle—known as the law of diffusion of gases—had already been discovered and analyzed by the Scottish chemist Thomas Graham. Snow brought the same logic to miasma: if there were poisonous elements floating in the air, emanating from the cesspools or the bone-boilers, they were likely to be so massively dispersed that they posed no health risk. (Snow was only half-right on this point, of course: the vapors proved irrelevant where epidemic disease was concerned, but they did in fact have long-term deleterious effects, in that many of the industrial fumes of the age were carcinogens.) Several years after the Broad Street epidemic, Snow would make this connection explicit, in a controversial appearance before one of Benjamin Hall’s public-health committees, defending the “offensive trades” (the bone-boilers, soap and dye makers, gut spinners) that stood accused of poisoning London’s air. “I have arrived at the conclusion,” Snow explained to the scandalized committee, “[that the offensive trades] are not injurious to the public health. I consider that if they were injurious to the public health they would be extremely so to the workmen engaged in those trades, and as far as I have been able to learn, that is not the case; and from the law of the diffusion of gases, it follows, that if they are not injurious to those actually upon the spot, where the trades are carried on, it is impossible they should be to persons further removed from the spot.” Call it the Sewer-Hunter Principle: if all smell truly was disease, then a scavenger descending into an underground tunnel of raw waste should be dead in seconds.

  Snow was also a doctor, a trained observer of physical symptoms, and he understood that the bodily effects of a disease were likely to offer important clues about the disease’s original cause. In the case of cholera, by far and away the most pronounced change in the body lay in the small intestine. The disease invariably began with that terrible expulsion of fluid
s and fecal matter; all the other symptoms followed from that initial loss of water. Snow couldn’t say exactly what kind of element was behind cholera’s catastrophic attack on the human body, but he knew from observation that it invariably launched that attack from one place: the gut. The respiratory system, on the other hand, was largely unaffected by cholera’s ravages. For Snow, that suggested an obvious etiology: cholera was ingested, not inhaled.

  Snow’s observational talents extended beyond the human body. The sad irony of his argument for the waterborne theory of cholera is that he had all the primary medical explanations in place by the winter of 1848–1849, and yet they fell on deaf ears for almost a decade. The tide eventually turned not because of his skills as a doctor or scientist. It wasn’t lab research that would ultimately persuade the authorities; it wasn’t direct observation of V. cholerae itself. It was Snow’s faithful, probing observation of urban life and its everyday patterns: the malt-liquor drinkers at the Lion Brewery; the late-night trips for cold water on hot summer nights; the tangled web of private water supplies in South London. Snow’s breakthroughs in anesthesia had revolved around his polymath skills as a physician, researcher, and inventor. But his cholera theory would ultimately depend on his skills as a sociologist.

  Equally important was the social connection Snow had to the subjects he observed. It is not an accident that of the dozens and dozens of cholera outbreaks that he analyzed in his career, the one for which he is most famous erupted six blocks from his residence. Like Henry Whitehead, Snow brought genuine local knowledge to the Broad Street case. When Benjamin Hall and his public-health committee made their triumphant appearance on the streets of Soho, they were little more than tourists, goggling at all the despair and death, and then retreating back to the safety of Westminster or Kensington. But Snow was a true native. That gave him both an awareness of how the neighborhood actually worked, and it gave him a credibility with the residents, on whose intimate knowledge of the outbreak Snow’s inquiry depended.

  Snow shared more than geography with the working poor of Golden Square, of course. While he had long since elevated himself in social status, his roots as the son of a rural laborer colored his perception of the world throughout his life—primarily in the sense of blocking out certain dominant ideas. Nowhere in Snow’s writings on disease does one ever encounter the idea of a moral component to illness. Equally absent is the premise that the poor are somehow more vulnerable to disease thanks to some defect in their inner constitution. Ever since he observed the Killingsworth mining outbreak as a young apprentice, Snow had long known that epidemics tended to afflict the lower orders of society. For whatever reason—probably some mix of rational observation and his own social awareness—that disparity led Snow to seek external causes, not internal ones. The poor were dying in disproportionate numbers not because they suffered from moral failings. They were dying because they were being poisoned.

  Snow’s resistance to the miasma theory was methodological as well. The strength of his model derived from its ability to use observed phenomena on one scale to make predictions about behavior on other scales up and down the chain. Observed failure of certain organ systems of the body could predict behavior of the whole person, which could in turn predict behavior in the social body en masse. If the symptoms of the cholera concentrated around the small intestine, then there must be some telltale characteristic in the eating and drinking habits of cholera victims. If cholera was waterborne, then the patterns of infection must correlate with the patterns of water distribution in London’s neighborhoods. Snow’s theory was like a ladder; each individual rung was impressive enough, but the power of it lay in ascending from bottom to top, from the membrane of the small intestine all the way up to the city itself.

  And so Snow’s immunity to the miasma theory was as overdetermined as the theory itself. Partly it was an accident of professional interest; partly it was a reflection of his social consciousness; partly it was his consilient, polymath way of making sense of the world. He was brilliant, no doubt, but one needed only to look to William Farr to see how easily brilliant minds could be drawn into error by orthodoxy and prejudice. Like all those ill-fated souls dying on Broad Street, Snow’s insight lay at the intersection point of a series of social and historical vectors. However brilliant Snow was, he would never have proved his theory—and might well have failed to concoct it in the first place—without the population densities of industrial London, or Farr’s numerical rigor, or his own working-class upbringing. This is how great intellectual breakthroughs usually happen in practice. It is rarely the isolated genius having a eureka moment alone in the lab. Nor is it merely a question of building on precedent, of standing on the shoulders of giants, in Newton’s famous phrase. Great breakthroughs are closer to what happens in a flood plain: a dozen separate tributaries converge, and the rising waters lift the genius high enough that he or she can see around the conceptual obstructions of the age.

  You can see the convergence of all these forces in Snow’s regimen that Wednesday. In the midst of the most important investigation of his life, he was still a working physician, managing the diffusion of gases. He delivered chloroform to two patients: one having hemorrhoids removed, the other having a tooth extracted. He spent the rest of the day in the streets of his neighborhood, probing, questioning, listening. Yet each conversation, however intimate, was shaped by the impersonal calculations of Farr’s statistics. He drew lines of connection between individual pathology and the wider neighborhood; he shifted perspective seamlessly from doctor to sociologist to statistician. He drew maps in his head, looking for patterns, looking for clues.

  HENRY WHITEHEAD DIDN’T POSSESS A THEORY OF CHOLERA of his own, but he’d been steadily knocking down other ones for days now. He knew that the well-to-do neighborhoods around Golden Square were abuzz with sneering explanations for the outbreak: the poor of Soho, on the mean side of Regent Street, had brought this upon themselves. Either their physical crisis was the embodiment of a moral crisis, a kind of divine retribution, or they had succumbed to the fear of disease, which in turn made the cholera more powerful over them. Whitehead had been stewing over these calumnies for days now, but his outrage reached a fever pitch when James Richardson, St. Luke’s scripture reader, failed to report for the noon vestry meeting. Richardson was one of Whitehead’s closest friends, a blustery former grenadier guard with a fondness for debating metaphysics late into the evening. Whitehead found him at his home, suffering from a cholera attack that had begun several hours before. Richardson recounted a conversation he had had with a frightened neighbor who had asked about the best antidote to ward off the cholera. “I don’t know what to take, but I do know what to do. It may neither prevent nor cure cholera; but it will save me from what is worse than cholera, i.e. from fear. I shall look up to my God, and though he slay me, yet I will trust in him.”

  If James Richardson—the very image of courage—could contract the disease, Whitehead thought, then the “inner constitution” explanation must certainly be false. With the number of new cases seemingly in decline, and with so much of the neighborhood emptied out, Whitehead finally had time to take stock of the situation, and he began thinking about ways to combat the popular prejudices. He was not a man of science, of course, but he knew as much about the path of the outbreak as did anyone, and perhaps if he wrote down his experiences, they would prove to be of some value to the wider population. Farr’s Weekly Returns, published in that morning’s Times, included this understated line: “On the north side of the Thames there has been a remarkable outbreak in the St. James District.” The abrupt nature of the description was almost an insult. The true story of the Golden Square outbreak had yet to be told.

  Richardson had mentioned one thing in passing that stuck with Whitehead as he returned to St. Luke’s. The scripture reader had drunk a glass of water from the Broad Street pump on Saturday, a day or two before his symptoms appeared. It was not his habit to drink from the pump, and Richardson wondere
d if perhaps there had been a connection between that glass and his subsequent illness. But Whitehead thought the connection unlikely. He had personally seen so many residents recover from cholera after drinking Broad Street water. He himself had enjoyed a glass a few nights before, and had thus far resisted the plague. Perhaps Richardson had drunk too little.

  WHAT WAS HAPPENING BELOW GROUND, IN THE DANK waters of the Broad Street well? The truth is, we don’t know. Clearly by Wednesday, it was significantly harder for the V. cholerae to find its way to a human small intestine, mostly because the number of people using the pump had dropped so precipitously, thanks to the death toll and the mass exodus. In that sense, it’s possible that the V. cholerae’s dramatic reproductive success over the weekend—think how many trillions of bacteria had been created in that short amount of time—had been the agent of its own demise. Establishing a base in a popular watering hole in London’s most densely populated district allowed the bacteria to spread through the neighborhood like wildfire, but the fire’s spread was so sudden and so extensive that it quickly burned through its primary fuel supply. There weren’t enough small intestines left to colonize.

  It’s also possible that the Vibrio cholerae had not been able to survive more than a few days in the well water below the Broad Street pump. With no sunlight penetrating the well, the water would have been free of plankton, and so the bacteria that didn’t escape might have slowly starved to death in the dark, twenty feet below street level. The purity of the well water may have played a role as well. V. cholerae much prefers water with a high saline content, or with extensive organic material. In distilled water, the organism dies off in a matter of hours. But the most likely scenario is that the bacterium was itself in a life-or-death struggle with another organism: a viral phage that exploits V. cholerae for its own reproductive ends the way V. cholerae exploits the human small intestine. One phage injected into a bacterial cell yields about a hundred new viral particles, and kills off the bacterium in the process. After several days of that replication, the population of V. cholerae might have been replaced by phages that were harmless to humans.