In the summer of 1854, cholera tore through the Golden Square neighborhood of London, decades before germ theory was accepted, and nearly 30 years before Robert Koch identified the cholera bacterium. The dominant medical theory of the day, miasma theory, blamed disease on foul-smelling “bad air” rising from rotting waste and sewage, not water. Snow suspected otherwise. He didn't need a lab to test it: he got a map of the neighborhood and marked a dot for every cholera death (Figure 1.1, below). Then he marked the neighborhood's water pumps.

A cluster of dots surrounded one pump on Broad Street. Snow interviewed residents and found that even people who lived closer to other pumps had been drinking from Broad Street because they preferred its taste. He also solved a puzzle. A nearby brewery had zero cholera deaths among its workers, because they drank their daily allowance of malt liquor instead of the water. Snow convinced local officials to simply remove the pump's handle. The outbreak was already slowing by then (many residents had fled the area), but it did not come back, and the pump handle became the most famous symbol in public health.

Snow then did something even more important for the future of epidemiology. He went back and compared cholera death rates between neighborhoods served by two different water companies, one of which drew water downstream of London's sewage and one upstream. Households served by the "downstream" company had a death rate more than 5 times higher. This is often called the first modern epidemiologic study: Snow counted cases, divided by the population at risk, and compared the rates between groups, the exact same logic Disease Detectives use today.

Did Snow disprove miasma theory? Not in his lifetime. His evidence was strong, but most officials weren't convinced: an 1855 government inquiry rejected his water theory, and miasma thinking hung on until germ theory (Pasteur's work in the 1860s, then Koch identifying the cholera bacterium in 1883) replaced it. Snow was right, but it took decades for the rest of medicine to agree.