John Snow and the Cholera Map

In the summer of 1854, a severe cholera outbreak struck the Soho district of London, killing over 600 people. Physician John Snow, skeptical of the prevailing "miasma" theory (that diseases were caused by bad air), painstakingly mapped the addresses of cholera victims and discovered they clustered around a single water pump on Broad Street. He persuaded local authorities to remove the pump handle, and the outbreak subsided. Snow's work — combining careful observation, data collection, and spatial analysis — is considered the founding act of modern epidemiology. He demonstrated that cholera was waterborne, not airborne, decades before germ theory was established. His methodology — tracing a disease to its source through data — remains the foundation of public health investigation.