John Snow

Snow traced the 1854 cholera outbreak in London's Soho to a single water pump on Broad Street by plotting cases on a map and interviewing residents, showing deaths clustered around the pump even among people who lived closer to other sources, while a workhouse and a brewery nearby whose residents drank from a different supply were spared. Having the pump handle removed did not by itself end an outbreak already declining, but the map and the door to door survey behind it stand as an early instance of tracing a disease to its physical source through data rather than accepting the prevailing miasma theory that bad air caused disease. Germ theory itself was not yet established; Snow's own explanation was inference from pattern, confirmed only later once the cholera bacterium was identified.