In the mid-1800s, London’s SoHo neighborhood was a terrible smelling place to live, as raw sewage ran through the gutters into open cesspools.

When a cholera epidemic broke out, 600 people died and many remaining residents fled, fearing – as was common then – that foul air spread the disease.

Local physician John Snow didn’t buy it. Ten years earlier, he had proposed that cholera spread through contaminated water. But lacking proof, he was ignored.

With an outbreak on his doorstep, he saw an opportunity to test his hypothesis – and save his neighborhood.

First, he obtained the addresses of every person who had died. Then he walked the streets plotting the deaths, and everything else, on a map.

Soon a pattern emerged. Most of the cases clustered around one public water well.

He began interviewing area residents. Locals with their own private wells had not gotten sick. Workers at a nearby pub, who drank a daily ration of beer instead of pump water, were also spared.

Snow convinced local authorities that water from the well was to blame, and persuaded them to remove the pump handle, forcing residents to draw their water elsewhere. Immediately the cholera outbreak ended.

With his cholera map, Snow had begun the field of evidence-based epidemiology. This was well before germ theory and microscopes could identify the true culprit, the cholera bacteria, that had spread through sewage leaking into the water supply.