During a serious outbreak of cholera in London in 1854, John Snow used a map to plot the locations of all the diarrheal cases he learned about. The source of the infection was unknown, but Snow thought that if the cases clustered geographically, he might gain a clue as to the actual source. Snow's map revealed a close association between the density of cholera cases and a single well located on Broad Street. Simply removing the pump handle of the Broad Street well ended the epidemic, proving that the well water was the source of infection.