Broad Street Pump Investigation

He mapped cholera deaths against pump locations, showing cases clustered around the Broad Street pump, and had its handle removed on 8 September 1854 after presenting his findings the day before.

Snow mapped the outbreak's cholera deaths against the locations of local water pumps, using what is now recognized as a Voronoi diagram to show that cases clustered around the Broad Street pump rather than any other. Over 300 people had died by 10 September 1854, including 127 people on or near Broad Street in the outbreak's first three days. Snow presented his findings to the parish's Board of Guardians on 7 September, and the pump handle was removed the following day, after which new cases fell off sharply.

It showed that comparing disease rates across defined groups, not theorizing about bad air, could trace a disease to its source, the method still central to epidemiology today.

It is regarded as epidemiology's founding event because it showed, before germ theory was accepted, that comparing disease rates across geographically and socially defined groups could trace a disease to its source. Beyond the pump map, Snow separately compared mortality between households served by different water companies and found a cholera death rate roughly fourteen times higher among those supplied by a sewage-polluted source, using data and comparison rather than the miasma, or bad-air, theory that dominated medical thinking at the time.