The disease John Snow traced to a contaminated water pump on Broad Street was cholera.
During the 1854 outbreak in Soho, London, Snow mapped cases and deaths around the Broad Street pump. He noticed that households using water from the pump had unusually high illness and mortality rates, while some nearby residents who obtained water elsewhere were largely spared. His evidence challenged the prevailing miasma theory, which blamed outbreaks on poisonous air.
Snow worked with local residents and the Reverend Henry Whitehead to identify the pump as the likely source. Authorities removed the pump handle, although the outbreak may already have been declining and the action did not by itself prove causation. Later investigations found that sewage contamination had introduced Vibrio cholerae into the water supply.
Snow's study is now regarded as a landmark in epidemiology because it combined geographic mapping, interviews, and comparison of exposure patterns. A common misconception is that he directly saw the cholera organism in the water; the bacterium was not identified until later, and Snow's original water examination did not conclusively demonstrate the pathogen.