The short story—that everyone knows—is that during a cholera outbreak in 1854 London, John Snow (the father of epidemiology) traced the source of the disease to a local water well. The pump handle was removed and the outbreak ended. That’s the short story. The long story is more interesting, but to better understand Snow’s accomplishment, and the context in which the epidemic occurred, it’s useful to consider two points:
1. The dominant belief about disease at the time was the miasma theory. By this view, a tainted, malodorous / befouled miasma caused disease: bad air made people sick. However, unlike how we may interpret the miasma theory from a modern viewpoint, they did not believe tainted atmosphere contained disease-causing microscopic particles. Instead, the rank air was itself diseased, and because of that, they were obsessed with smell. Hospitals kept dishes of bleach or vinegar or other deodorants out to reduce the smell and eliminate the noxious and unhealthy miasma; public health projects often similarly involved large amounts of deodorizing and little actual disinfecting. To the miasmatist, if the smell could be eliminated, the danger was gone. That was a problematic, and as Snow discovered, very deeply entrenched viewpoint.
2. London was shitty. I mean that literally, it was full of shit. Partially this was a byproduct of rapid growth; the population had nearly tripled from 1800-1850 (and there were, of course, no high-rise apartment complexes; the population density must have been oppressive). By the 1840s, some rudimentary sewer systems had been installed, but for the many houses not plugged in to that system or for whom the sewers had not yet reached, waste was collected in cesspools. The cesspools were usually in the backyard or the cellar (with luck, below the floorboards; when full, very much not below the floorboards). Ideally, when the cesspool filled, you called a cleanup crew to come and load all that shit into barrels and buckets and take it out to the farmland. As London became more urban and sprawling, those farmlands became more and more remote, hauling waste became more expensive, the shit, inevitably, became more mountainous. In a great Victorian turn of phrase, waste was called “night soil”: “Call the night soil man”, you’d say, when you ran out of room to store your filth. For most people, the only fiscally viable solution was just to let the shit accumulate in their cellars, backyards, or wherever else it could be dumped. Yes, it was exactly as unpleasant as it sounds. Mid-19th century London is not high on my list of places to go when I acquire a time machine.
The pupu platter of miasma theory and London’s overcrowding/human waste problems really hit the fan in the 1840s. The miasmatist solution to the unhealthy atmosphere caused by the proliferation of cesspools was a primitive sewer system. That should have been a really good idea, except that the sewers emptied into the Thames, turning what was once a pristine, commercially fished river into a horrible pit of sewage and sadness. This was, of course, a-ok to the miasma-believers, because the smell was out of your house, right? They weren’t entirely wrong—not living near piles of human waste is generally a good idea—but ran afoul of that newfangled invention: running water. For some companies, the intake pipes pulled water from sewage-rich waters of the Thames, and this alone was probably responsible for hundreds or thousands of cholera cases throughout the years. Running water also exacerbated the waste disposal issue, as flushing toilets produced far more waste than simple chamberpots. Of course, despite all this, it would end up being the lack of sewers that was responsible for the Broad Street outbreak.
• • •
When cholera hit the Soho neighborhood, it hit hard. In the first three days, 127 people died—this in a several-block radius—and more than 500 would eventually succumb. Most of those who didn’t die had fled the neighborhood. Snow, who lived nearby, had five years earlier theorized that cholera may be waterborne. His evidence was circumstantial at the time, but now he had an opportunity to investigate further.
His efforts to track down the source were prodigious. He knocked on several hundred doors in a cholera-ridden neighborhood, collecting what information (and water samples) he could from survivors. He found two classes of people: those who drank water from the Broad Street pump and got sick, and those who hadn’t drunk the water and remained healthy. This delineation could be jarring: one factory close to the pump kept large barrels of pump water for their employees to drink, most of whom contracted cholera and died. Employees at a brewery even closer to the pump were largely spared because they mostly drank beer (in fact, beer and wine may have been historically popular because of their anti-microbial properties). Snow had what neuropsychologists would call a double dissociation: not just uni-directional correlations showing that drinking the bad pump water was more likely to lead to cholera, but also the converse—that avoiding it meant avoiding infection. His initial evidence was enough for the city council to remove the pump handle, and the outbreak abated.
Ultimately, Snow and his assistant (a local priest who was initially a proponent of the miasma theory until being swayed by Snow’s evidence) uncovered the source of the infection and the index case: a cesspit near the public water pump was contaminating the water. Given the rigorous study and seemingly iron-clad evidence, you’d think Snow would have put the miasma theory to bed. But like so many other people faced with disconfirming evidence, it only made the miasmatists believe harder.
Snow’s ideas were largely ignored in his own time: when he died several years later, it was his work with controlling dosage of ether for anaesthetic purposes, and not his epidemiological insights, that were mentioned in his obituary. It would be several decades, several thousand more deaths from cholera, and one particularly pungent summer (the “big stink”, when a confluence of stagnant sewage-ridden water on the Thames and high temperatures made London smell to beat the band) before a proper sanitation system was put into place In London. It was several decades before the germ theory of disease came into vogue. (Somewhat ironically, an Italian scientist had isolated the cholera-causing bacteria under a microscope in the 1840s, but no one—including Snow—recognized it)
It’s almost hard to comprehend what Snow did. Consider that, at the time of the Broad Street outbreak: a) epidemiology didn’t exist, b) the germ theory of disease didn’t exist, and c) because cholera bacteria had not been isolated, he had no way to directly test his ideas. To deduce that cholera was waterborne without the benefit of a germ theory of disease is impressive enough, but to then understand how to gather the data that would support that theory (and refute miasma theory) is a staggering feat of intellect. For example, his famous map generally shows that mortality increased for people closer to the pump, but he was clever enough to recognize that the miasmatists might argue simply that the pump was expelling diseased air. So he then demonstrated that it was the distance by foot to the pump that mostly strongly predicted infection, and not the distance as the crow flies. Given that a miasma should not be restricted only to walking paths, it’s a crucial piece of evidence against the theory.
• • •
There’s something especially nefarious when an idea is partially right, but not for the reasons we think. In the book, Johnson lays out a series of socio-cultural biases that allowed miasma theory to propagate, but a big part of its acceptance is that it’s not entirely wrong. Some diseases are transmitted through the air, so if you went looking for evidence supporting miasma theory, you could find it. Even for diseases not transmitted through the air, the pattern of infection often hints at an environmental (i.e., atmospheric) cause, and there’s little reason to be swayed from that viewpoint—after all, the pump could be expelling noxious air, not diseased water.
But being kind of right can sometimes be even more dangerous than being outright wrong. A theory that’s baseless is easy to disprove; a theory that’s partially right and matches some intuitions about how the world works becomes entrenched much more easily. And then you get all these other unforeseen consequences: the public health commissioner was on the right track when he realized that it might not be altogether healthy for people to be living on top of cesspools, and went about installing a sewer system. But the miasmatist just wants the bad air to be somewhere else, and didn’t have any intellectual framework for recognizing that even if you couldn’t smell the shit, it could still do its damage. It did so most prominently in London and Chicago, where presumably “clean” water was drawn in from the same place the sewers let off, until the folly of that approach was laid bare.