John Snow and the Broad Street Pump

During a cholera outbreak in London's Soho district in 1854, physician John Snow suspected contaminated water, not bad air, was to blame. He mapped deaths and found they clustered around a public water pump on Broad Street. Snow persuaded local officials to remove the pump's handle on 8 September 1854, by which time the outbreak was already declining. He also compared households supplied by two water companies, one drawing water from a sewage-polluted stretch of the Thames, and found far higher death rates among its customers.

Local curate Henry Whitehead, at first skeptical, investigated and found evidence supporting Snow, including a cesspool near the well. Snow's ideas were not widely accepted in his lifetime, but his methods of mapping, comparing groups and testing a hypothesis became foundations of epidemiology, the study of how disease spreads.