During the 1800's, England suffered regular epidemics of Asiatic cholera that killed thousands of its residents.  Opinions in the medical community regarding the origins of the disease were divided, and, as such, no effective prevention or treatment was implemented.

Physician John Snow set out to prove his theory that cholera was a disease spread by unsanitary water.  During the 1854 outbreak, he began an epidemiological study of the area and residents around the public water pump on Broad Street in London.  Snow tracked hundreds of cases of cholera to nearby schools, restaurants, businesses and pubs.  By mapping the residences of those who died from the outbreak in proximity to the Broad Street pump and representing the fatalities as short lines at those addresses, Snow developed positive proof that the pump was the source of the cholera epidemic.

For additional information, see "John Snow - a historical giant in epidemiology" on the UCLA Department of Epidemiology Web site http://www.ph.ucla.edu/epi/snow.html.  The included map is courtesy of that site.