London’s 1854 cholera outbreak was halted when John Snow, the father of modern epidemiology, traced the source of the disease to a water well. The pump handle was removed and the outbreak ended. That’s the short story. Two critical factors play into the longer, and more interesting, story of Snow and the cholera outbreak.
1. The dominant belief about how disease spread was miasma theory, which held that bad, tainted air made people sick. We know now that some diseases can be transmitted through the air, but at that time germ theory did not exist, so miasma theory was not a belief that microscopic disease-causing particles could be found in the air. Instead, rank air itself was diseased. Consequently, physicians and public health officials were obsessed with smell. Hospitals kept dishes of bleach or vinegar or other deodorants out to reduce the smell, thus eliminating the befouled air. Public health projects usually involved deodorizing but no disinfecting. To a miasmatist, if the smell was eliminated, the danger was gone.
2. London was shitty. I mean that literally: it was full of shit, a consequence of rapid growth (the population tripled from 1800-1850) and poor sewage infrastructure. By the 1840s, some rudimentary sewer systems were installed, but for the vast majority of families not plugged into it, waste was collected in cesspools. The cesspools were commonly in the backyard or the cellar—hopefully below the floorboards, but when full, very much not below the floorboards. When a cesspool filled, a cleanup crew would load the waste into barrels and haul it to some farmland. But as London became more urban and sprawling, farmland became more and more remote, and hauling waste (“night soil” in Victorian parlance) became more and more expensive. For many people, waste simply accumulated in their cellars and backyards, which is exactly as unpleasant as it sounds. Mid-19th century London is not high on a list of places to go when acquiring a time machine.
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The combination of miasma theory and London’s overcrowding and waste problems hit the fan in the 1840s. The miasmatist solution to the proliferation of overfilled cesspools was a primitive sewer system—which was a good idea in principle, except that the sewers emptied into the Thames, turning it into a sewage pit. But for a miasma-believer, the smell in one’s house was less rank, and thus less dangerous. This belief wasn’t entirely wrong, but ran headlong into a newfangled invention: running water. For those new companies supplying water to Londoners, intake pipes pulled water straight from the Thames, where the sewers deposited their cargo. Running water also exacerbated the waste disposal issue, as flushing toilets produced more waste volume than chamberpots. Before the outbreak, these factors were probably responsible for hundreds or thousands of cholera cases.
Despite the minefield of primitive sewer and water systems, ultimately the 1854 outbreak was caused by a lack of sewers. Cholera hit the Soho neighborhood quickly and violently: in the first three days, 127 people died in a several block radius. More than 500 would eventually perish. Hundreds more fled the neighborhood in terror.
Snow, a physician, had theorized five years earlier that cholera was a waterborne disease, and the outbreak near him was an opportunity to prove it. He knocked on hundreds of doors in cholera-ridden neighborhoods, collecting information and water samples from survivors. Two classes of people emerged: those who drank from the Broad Street pump and got sick, and those who hadn’t drunk from it and remained healthy. The delineation could be jarring; one factory near the pump pulled drinking water from there, and the majority of their employees were sickened, whereas employees at a factory even nearer the pump were healthy because they mostly drank beer. Snow had what neuropsychologists call a double dissociation: not just evidence that drinking pump water led to cholera, but also the converse—that avoiding it meant avoiding illness. The initial evidence was enough for the city council to remove the pump handle, and the outbreak abated.
Snow and his assistant kept going, ultimately showing that a cesspit near the pump was contaminating the water. They even found the index case, though it wasn’t called that at the time because epidemiology didn’t yet exist. (Snow’s assistant, by the way, was a priest and miasmatist who was compelled by the evidence to give up on miasma theory). Rigorous study and iron-clad evidence should have put miasma theory to bed, but old theories die hard, even when faced with disconfirming evidence.
When Snow died several years later, it was his work with ether as an anaesthetic, and not his epidemiological insights, that were mentioned in his obituary. It was several decades, several thousand more deaths from cholera, and one pungent summer (the “big stink,” a summer of high temperatures, stagnant sewage on the Thames, and ghastly smells) before a proper sanitation system was put into place In London. And it was several decades before germ theory was popularized.
It’s almost hard to comprehend what Snow did. When he studied the outbreak, neither epidemiology nor germ theory even existed; since cholera bacteria hadn’t been isolated, he had no way to directly test his hypotheses. There was no intellectual framework for what he was doing; he had to deduce that cholera was waterborne without benefit of the concept of germs, and work out how to gather data to support that theory. For example, his famous map showed that mortality increased closer to the pump. But miasmatists might argue that the pump was expelling diseased air, which would cause the same pattern. So Snow showed that it was the distance by foot to the pump that most strongly predicted infection, not actual distance. It was a staggering act of intellect; stepping entirely outside established paradigms and beliefs, then generating entirely new hypotheses and tests that turned out to be right.
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Reading about the Broad Street outbreak, I keep thinking about how it can be especially nefarious when an idea is partially right. Miasma theory was so well accepted in part because it’s not entirely wrong. Some diseases are transmitted through the air, and so there’s plenty of evidence to support the idea of a sickness-causing miasma. And as Snow found out with his map, even for diseases that aren’t transmitted through the air, the pattern of infections is still suggestive of environmental or atmospheric causes, because people move around just as air does.
Baseless theories are easy to disprove; but a theory that’s partially right and matches some intuitions about how the world works is easily entrenched and difficult to eliminate. The public health commissioner in London had the fundamental part right when he had sewer systems installed: it’s not healthy for people to live on top of cesspools. But a mismatist worries only about getting the bad air somewhere else, and has no intellectual framework for recognizing that even if you can’t smell the shit, it may still do its damage—as it did in London and Chicago, where clean water was drawn downstream from sewage runoff. Being kind of right can sometimes be more dangerous than being outright wrong.