When a large outbreak occurred in London in 1854, Dr. John Snow collected data about the individual cases, especially their location in Soho (London), using nascent methods of spatial analysis and contact tracing to conclude that contaminated water was the disease vector, and successfully had the source shut off. The map that accompanied his 1855 report showed individual cases, stacked at each house location, clearly showing a concentration around the Broad Street Pump as well as gaps in locations that had other water sources. The map is now hailed as revolutionary; although its role in the investigation itself and its effect on settling the debate over the cause of the disease is often overstated, it does deserve recognition for Snow's insight that a map was the most effective tool for communicating the spatial patterns of the disease.