The Ghost Map

by Steven Johnson

  In lay terms, the human brain appears to have evolved an alert system whereby a certain class of extreme smells triggers an involuntary disgust response that effectively short-circuits one’s ability to think clearly—and produces a powerful desire to avoid objects associated with the smell. It is easy to imagine the evolutionary pressures that would bring this trait into being. Once again microbes are at the center of the story. Eating meat or vegetation that has already begun the decomposition process poses a significant health risk, as does eating foods that have been contaminated with fecal matter—precisely because of the microbial life-forms that are doing the decomposing. Putrefying foods release several organic compounds into the air; they have names like putrescine and cadaverine. Bacteria recycling energy stored in fecal matter releases hydrogen sulfide into the air. Disgust at the scent of any of these compounds is as close to a universal human trait as we know. You can think of it as a form of evolutionary pattern recognition: over millions of years of evolution, natural selection hit upon the insight that the presence of hydrogen sulfide molecules in the air was a reasonably good predictor that microbial life-forms that could be dangerous if swallowed were nearby. And so the brain evolved a system for setting off an alarm whenever those molecules were detected. Nausea itself was a survival mechanism: it was better to void the contents of your stomach than run the risk that the smell was coming from the antelope you’d just finished eating.

  But those telltale molecules—hydrogen sulfide, cadaverine—were clues pointing to a threat. They were not the threat itself. If you press your nose up against a decomposing banana or antelope, you might well make yourself vomit, but you won’t contract a disease from the experience, however repulsive. Breathing in pure methane gas or hydrogen sulfide could kill you, of course, but bacterial decomposition doesn’t release anywhere near enough of these gases to saturate the environment. In other words, methane and putrescine and cadaverine are the smoke. Microbes are the fire.

  Basing the alarm system around smell made perfect sense for the environmental conditions of the hunter-gatherer lifestyle. The smell of decay and fecal waste was relatively rare in a world where humans lived in small roving bands; there were no sewers or dustheaps on the savannahs of Africa, precisely because the hunter-gatherers had such low population densities and such mobile lifestyles. You could just leave your waste behind and move on to a new spot; odds were, the bacteria would have recycled it all by the time another human returned. The alarm system of disgust likely evolved both because the threat posed by eating decaying organic matter was a serious one, and because the smell that signaled the presence of decaying matter was unusual. If the smell had been ubiquitous—if, say, some common African flower had begun emitting hydrogen sulfide from its blooms—then the human brain might have evolved another way of anticipating the presence of decaying food.

  The trouble is that survival strategies optimized for a hunter-gatherer lifestyle play out differently in a modern city of two million people. Civilization had produced many transformations in the experience of human life: farms, wheels, books, railroads. But civilized life had another distinguishing feature: it was a lot smellier. Densely packed populations of people without modern waste-management systems produced powerfully repellent odors. When Mayhew describes his repulsion at the smell of hydrogen sulfide on the streets of Bermondsey, you can see in the passage a clash between three distinct epochs somehow struggling to share the same space: an industrial-era city with an Elizabethan-era waste-removal system as perceived by a Pleistocene-era brain.

  The miasmatists had plenty of science and statistics and anecdotal evidence to demonstrate that the smells of London weren’t killing people. But their gut instincts—or, more like it, their amygdalas—kept telling them otherwise. All of John Snow’s detailed, rigorous analysis of the water companies and the transmission routes of the Horsleydown outbreak couldn’t compete with a single whiff of the air in Bermondsey. The miasmatists were unable to override the alarm system that had evolved so many aeons before. They mistook the smoke for the fire.

  MIASMA’S HEGEMONY HAD ONE OTHER BIOLOGICAL BASIS. Our noses are far more adept than our eyes at perceiving the very small. It takes only a few molecules of cadaverine attaching to the olfactory receptors in your upper nasal passages for you to become aware of the smell of decay. But your eyes are useless at the scale of molecules. In many respects, human visual perception is unrivaled among earth’s life-forms—the legacy of a nocturnal mammal who needed to forage and hunt in the dark. But molecules remain several orders of magnitude below the threshold of human visual perception. We can’t see most ordinary cells that those molecules build, even whole populations of cells. A hundred million V. cholerae floating in a glass of water would be invisible to the naked eye. Microscopes had been in use for more than two centuries, and while a few isolated researchers had caught a glimpse of microbes in their labs, the existence of a bacterial microcosmos was still the stuff of fantasy and conjecture for the mid-Victorian mind. But the stench of decomposition was all too real. Smelling was believing.

  The miasma theory drew on other sources for its power as well. It was as much a crisis of imagination as it was pure optics. To build a case for waterborne cholera, the mind had to travel across scales of human experience, from the impossibly small—the invisible kingdom of microbes—to the anatomy of the digestive tract, to the routine daily patterns of drinking wells or paying the water-company bills, all the way up to the grand cycles of life and death recorded in the Weekly Returns. If you looked at cholera on any one of those levels, it retreated back into the haze of mystery, where it could be readily rolled back to the miasma theory, given the pedigree and influence of miasma’s supporters. Miasma was so much less complicated. You didn’t need to build a consilient chain of argument to make the case for miasma. You just needed to point to the air and say: Do you smell that?

  And of course there were more than a few instances where the statistical evidence did in fact seem to stack the odds in miasma’s favor. Neighborhoods with unsanitary water supplies generally suffered from poor air quality as well; many of them lay at the lower elevations that Farr relentlessly documented in his Weekly Returns. For every sewer-hunter living happily into his sixties, there were a hundred false positives dying in the low elevations of Bermondsey.

  Raw social prejudice also played a role. Like the other great scientific embarrassment of the period—phrenology—the miasma theory was regularly invoked to justify all sorts of groundless class and ethnic biases. The air was poisoned, to be sure, but the matter of who fell ill, and what disease they suffered from, was determined by the constitution of each individual breathing in the air. So went Thomas Sydenham’s internal-constitution theory of the epidemic, an eccentric hybrid of weather forecasting and medieval humorology. Certain atmospheric conditions were likely to spawn epidemic disease, but the nature of the diseases that emerged depended partly on a kind of preexisting condition, a constitutional susceptibility to smallpox, or influenza, or cholera. The distinction was often defined as one between exciting and predisposing causes. The exciting cause was the atmospheric condition that encouraged a certain kind of disease: a specific weather pattern that might lead to yellow fever, or cholera. The predisposing cause lay in the bodies of the sufferers themselves. That constitutional failing was invariably linked to moral or social failing: poverty, alcohol abuse, unsanitary living. One alleged expert argued in 1850: “The probability of an outburst or increase during [calm, mild] weather, I believed to be heightened on holidays, Saturdays, Sundays, and any other occasions where opportunities were afforded the lower classes for dissipation and debauchery.”

  The idea of one’s internal constitution shaping the manifestation of disease was not just useful for affirming social prejudices about the moral depravity of the lower classes. It also helped paper over a massive hole in the theory itself. If the miasma seemed unusually capricious in its choice of victims for poison allegedly circulating in the atmosphere—
if it killed off two housemates but left the remaining two unscathed despite the fact that they were all breathing the same air—the miasmatists could simply point to the differences in constitution between the victims and the survivors to explain the disparity. Although the poisonous vapors were distributed equally through the environment, each inner constitution possessed its own distinct vulnerability.

  Like much of the reasoning that lay behind the miasma theory, the idea of an inner constitution was not entirely wrong; immune systems do vary from person to person, and some people may indeed be resistant to epidemic diseases like cholera or smallpox or plague. The scaffolding that kept miasma propped up for so long was largely made up of comparable half-truths, correlations mistaken for causes. Methane and hydrogen sulfide were in fact poisons, after all; they just weren’t concentrated enough in the city air to cause real damage. People were more likely to die of cholera at lower elevations, but not for the reasons Farr imagined. And the poor did have higher rates of contagion than the well-to-do, but not because they were morally debauched.

  Yet miasma had just as much to offer the liberals as it did the conservatives. Chadwick and Nightingale and Dickens were hardly bigots where the working classes were concerned. Miasma, for them, was not a public sign of the underclasses’ moral failing; it was a sign of the deplorable conditions in which the underclasses had been forced to live. It seemed only logical that subjecting such an immense number of people to such deplorable living environments would have a detrimental effect on their health, and of course, the liberal miasmatists were entirely right in those basic assumptions. Where they went wrong was in assuming that the primary culprit lay in the air.

  And so, on August 29, when the Morning Chronicle welcomed Benjamin Hall to his new job as president of the Board of Health, the editors included more than a few cutting remarks at the expense of Edwin Chadwick; yet they embraced the theory of miasma with both arms and urged the new president to continue the work of enforcing the Nuisances Removal and Contagious Diseases Prevention Act. There may be no clearer example of miasma’s dark irony: on the very day that the outbreak in Golden Square was beginning, one of London’s most prestigious papers was urging the Board of Health to accelerate its work poisoning the water supply.

  MIASMA TURNS OUT TO BE A CLASSIC CASE OF WHAT FREUD, in another context, called “overdetermination.” It was theory that drew its persuasive power not from any single fact but rather from its location at the intersection of so many separate but compatible elements, like a network of isolated streams that suddenly converges to form a river. The weight of tradition, the evolutionary history of disgust, technological limitations in microscopy, social prejudice—all these factors colluded to make it almost impossible for the Victorians to see miasma for the red herring that it was, however much they prided themselves on their Gradgrindian rationality. Every research paradigm, valuable or not, in the history of ideas has been buttressed by a comparable mix of forces, and in this sense the deconstructionists and the cultural relativists—so often the subject of mockery lately—have it right to a certain extent, though they tend to place undue stress on purely ideological forces. (Miasma was as much a creature of biology as of politics.) The river of intellectual progress is not defined purely by the steady flow of good ideas begetting better ones; it follows the topography that has been carved out for it by external factors. Sometimes that topography throws up so many barricades that the river backs up for a while. Such was the case with miasma in the mid–nineteenth century.

  But most of these dams eventually burst. Yes, the path of science works within regimes of agreement and convention, and history is littered with past regimes that were overthrown. But some regimes are better than others, and the general tendency in science is for explanatory models to be overthrown in the name of better models. Oftentimes because their success sows the seeds of their destruction. Miasma became so powerful that it inspired a massive, state-sponsored intervention in the daily lives of millions of people, clearing the air by draining the cesspools. That intervention, miscalculated as it was, had the paradoxical effect of making the patterns of the epidemic more visible, at least to eyes that were capable of seeing them. And seeing the patterns more clearly means progress, in the long run at least.

  JOHN SNOW SPENT MOST OF TUESDAY SEARCHING FOR patterns. In the morning he was knocking on doors, interrogating strangers in the street, asking anyone he encountered for anecdotal evidence about the outbreak and its victims. The clues he found were tantalizing, but too many doors went unanswered, and the dead couldn’t report on their recent drinking habits. Personal testimony would not take him far in an evacuation zone. And so at midday he paid a visit to the Registrar-General’s Office, where Farr gave him an early look at the numbers being calculated for the week. Eighty-three deaths had been reported in Soho between Thursday and Saturday. Snow asked for a complete list, including addresses, and returned to Broad Street to continue his sleuthing. He stood at the base of the pump, and ran through the addresses on the list. From time to time, he gazed out at the empty streets around him, imagining the paths the residents might take to find their way to water.

  It was going to take more than body counts to prove that the pump was the culprit behind the Broad Street epidemic. Snow was going to need footprints, too.

  “BLUE STAGE OF THE SPASMODIC CHOLERA”

  Wednesday, September 6

  BUILDING THE CASE

  A HUNDRED YARDS WEST OF THE BROAD STREET PUMP, IN the dark alley of Cross Street, a tailor lived in a single room at number 10, sharing the space with his five children, two of whom were fully grown. On warm summer nights the heat in their cramped living space could be unbearable, and the father would often wake after midnight and send one of the boys out to fetch some cool well water to combat the sweltering air. They lived only two blocks from the pump at Little Marlborough Street, but that water had such an offensive smell that they regularly walked the extra block to Broad Street.

  The tailor and his twelve-year-old boy had been struck in the first hours of the outbreak, and both were dead by Saturday. Snow had found their address listed in the inventory of deaths that Farr had supplied him. Several other deaths were recorded on Cross Street as well. The location had caught Snow’s eye when he first arrived back at the pump to survey the surrounding streets, armed with the addresses of the dead. Almost half the deaths Farr had recorded were linked to addresses within his line of sight; and half the remaining ones came from residences that were only a matter of steps from Broad Street itself. The Cross Street deaths were unusual, though: to make it to the Broad Street pump from there, you had to wind your way through two small side streets, then take a right onto Marshall Street, then another left, and then walk a long block down Broad Street. To get to the Little Marlborough pump, though, you simply strolled down the alley, walked two short blocks north, and you were there. It was within your line of sight if you stood at the very end of Cross Street.

  Snow had noticed another element while scanning Farr’s records: the deaths on Cross Street were much less evenly distributed than the ones in the immediate vicinity of the pump. Almost every house along Broad Street had suffered a loss, but there were only a handful of isolated cases on Cross Street. This is what Snow was looking for now. He could see at a glance that he’d be able to demonstrate that the outbreak was clustered around the pump, yet he knew from experience that that kind of evidence, on its own, would not satisfy a miasmatist. The cluster could just as easily reflect some pocket of poisoned air that had settled over that part of Soho, something emanating from the gulley holes or cesspools—or perhaps even from the pump itself. Snow knew that the case would be made in the exceptions to the rule. What he needed now were aberrations, deviations from the norm. Pockets of life where you would expect death, pockets of death where you would expect life. Cross Street was closer to Little Marlborough, and thus should have been spared in the outbreak, according to Snow’s theory. And indeed, it had largely been spared, but
for the four cases Farr had reported. Could those cases have some connection to Broad Street?

  Sadly, by the time Snow arrived at 10 Cross to interview the tailor’s surviving children, he was too late. He learned from a neighbor that the entire family—five children and their father—had died in the space of four days. Their late-night thirst for Broad Street water had destroyed them all.

  IN HIS MIND SNOW WAS ALREADY DRAWING MAPS. HE’D imagined an overview of the Golden Square neighborhood, with a boundary line running an erratic circle around the Broad Street pump. Every person inside that border lived closer to the poisoned well; everyone outside would have had reason to draw water from a different source. Snow’s survey of the neighborhood, based on Farr’s initial data, revealed ten deaths that lay outside the boundary line. Two of them were the tailor and his son on Cross Street. After a few hours of conversation, Snow determined that three others were children who went to school near Broad Street; their grieving parents reported that the children had often drunk from the pump on their way to and from school. Relatives confirmed that three other casualties had maintained a regular habit of drawing water from Broad Street, despite living closer to another source. That left two remaining deaths outside the border with no connection to Broad Street, but Snow knew that two cholera deaths over a weekend was well within the average for a London neighborhood at that time. They might easily have contracted the disease from a different source altogether.