During the 1854 cholera outbreak in London's Soho, the physician John Snow mapped the addresses of the dead and traced an intense cluster to a single public water pump on Broad Street. By correlating cases with the contaminated water source rather than 'bad air', he built the case that cholera was waterborne and transmitted through sewage-tainted supply. Local officials removed the pump handle, and the outbreak subsided. Snow's spatial analysis is a foundational piece of disease cartography.
The investigation helped found modern epidemiology and shifted the scientific basis of sanitation from miasma theory toward waterborne transmission. It strengthened the practical case for clean water, comprehensive sewerage and density control that drove the great urban sanitary engineering projects of the later nineteenth century.