During the cholera epidemics of the 19th century, there were many scientists who tried to understand the precise causes of the disease. One of these scientists, John Snow, was particularly influential in the history of epidemiology (the study of how diseases spread). Snow was a model scientist, combining exhaustive, hands-on research with conceptual rigor to produce a strong, testable theory of the causes of cholera. However, Snow faced many challenges in studying cholera. Unlike many other scientists, he couldn’t conduct experiments isolating the causes and effects of the disease; instead, he had to rely upon case studies and statistical analyses of past outbreaks. Throughout his investigations, Snow exemplified the process by which good scientists should try to understand the world.
Perhaps Snow’s most valuable quality as a scientist was his willingness to “do the work,” putting in countless hours to develop a workable hypothesis about the causes of cholera. In spite of his successful career as an anesthesiologist, Snow continued to research unexplored areas of science, including epidemiology. After the cholera outbreak of 1848, Snow began to study the neighborhoods where the disease had been most common. Snow didn’t have the luxury of conducting experiments about cholera; instead, he interviewed dozens of people who’d lost loved ones to cholera, and pored over international reports on the spread of the disease. In the process of researching cholera, Snow developed a hypothesis: cholera was waterborne. By spending so much time studying cholera patients, Snow ensured that his hypothesis would be strong and supported by all the data. He also freed himself from some of the scientific community’s misconceptions about cholera—in particular, that it was spread by bad smells.
During the 1854 cholera epidemic, Snow tested his hypothesis. In the process, he illustrated the features of a good scientific theory. First, a good scientific theory should explain a real-world phenomenon: Snow’s waterborne theory succinctly explained why a cholera epidemic had broken out in Soho in 1854—the well beneath the Broad Street pump, where thousands of Soho residents got their water, had been contaminated. A good scientific theory should also be “strong” in the sense that it can describe many different cases by analyzing a relatively small number of causes. While other scientists of the era developed highly complicated theories to explain why certain people contracted cholera and others didn’t (often attributing the difference to intangible factors such as “internal constitution”), Snow’s theory used a single factor (the water from Broad Street) to explain why some people got cholera. Snow could even explain why a woman named Susannah Eley, who didn’t even live in London, had contracted cholera at the time of the outbreak—Susannah’s sons, the Eley brothers, had sent her a glass of water from the Broad Street pump. Perhaps the most valuable aspect of Snow’s waterborne theory—and of any good scientific theory—was its falsifiability. A good theory can be tested and disproven—otherwise, there’s no way for other scientists to know if it’s right or wrong. If cholera were waterborne, as Snow believed, then one would expect that the people in Soho who didn’t drink water wouldn’t get sick. Sure enough, the employees of the local brewery, who were paid in alcohol, didn’t contract cholera in 1854. Finally, Snow’s scientific theory also pointed the way toward further research—it answered questions, but also posed new questions for future scientists to study. Snow’s theory explained why certain people did and didn’t contract cholera, but it didn’t address what cholera itself was–only that it traveled through water. It took many years for scientists to determine the “agent” of cholera (i.e., the cholera bacterium).
In the end, Johnson suggests, John Snow’s greatest contribution to the history of science wasn’t the waterborne theory of cholera itself (which had actually been proposed twenty years previously, and ignored)—rather, it was Snow’s willingness to conduct research in order to arrive at a strong, falsifiable theory of disease. In modeling the scientific process so perfectly, Snow inspired other scientists to take after his example, and proved that science is often the most effective tool for responding to a crisis.
The Scientific Process ThemeTracker
The Scientific Process Quotes in The Ghost Map
For Londoners, the specific menace of cholera was a product of the Industrial Age and its global shipping networks: no known case of cholera on British soil exists before 1831.
One British doctor, Thomas Latta, hit upon this precise cure in 1832, months after the first outbreak, injecting salty water into the veins of the victims. Latta's approach differed from the modern treatment only in terms of quantity: liters of water are necessary to ensure a full recovery. Tragically, Latta's insight was lost in the swarming mass of cholera cures that emerged in the subsequent decades.
Snow was a truly consilient thinker […] Snow’s work was constantly building bridges between different disciplines, some of which barely existed as functional sciences in his day, using data on one scale of investigation to make predictions about behavior on other scales.
Snow also recognized the weakness of the contagionist argument. […] Clearly, the cholera was not communicated through sheer proximity. In fact, the most puzzling element of the disease was that it seemed capable of traveling across city blocks, skipping entire houses in the process.
Some of those forces were ideological in nature, matters of social prejudice and convention. Some revolved around conceptual limitations, failures of imagination and analysis. Some involve the basic wiring of the human brain itself. Each on its own might not have been strong enough to persuade an entire public-health system to empty raw sewage into the Thames. But together they created a kind of perfect storm of error.
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.
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.
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.
Snow's argument was persuasive—and, besides, they had few other options. If Snow was wrong, the neighborhood might go thirsty for a few weeks. If he was right, who knew how many lives they might save? And so, after a quick internal consultation, the Board voted that the Broad Street well should be closed down.
Hall's list is a kind of straitjacket for an eventual document. You can tell from just scanning the instructions what kind of document they will ultimately produce: a rich and impossibly detailed inventory of the smells of Soho circa 1854.
Standing in front of his haggard parishioners in the half-empty church, he noted the disproportionate number of poor, elderly women in the pews. He congratulated them on their "remarkable immunity from the pestilence." But even as he spoke the words, he wondered: How can this be? What kind of pestilence spares the old and the destitute?
This is circular argumentation at its most devious. The committee begins with the assertion that cholera is transmitted via the atmosphere. When it discovers evidence that contradicts this initial assertion—a clear case that cholera has been transmitted by water—the counter-evidence is invoked as further proof of the original assertion: the atmosphere must be so poisoned that it has infected the water as well.
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 global challenges that we face are not necessarily an apocalyptic crisis of capitalism or mankind’s hubris finally clashing with the balanced spirit of Gaia. We have confronted equally appalling crises before. The only question is whether we can steer around these crises without killing ten million people, or more. So let’s get on with it.