How did John Snow trace the 1854 London cholera outbreak to the Broad Street pump?

During the 1854 cholera outbreak in London, John Snow challenged the prevailing miasma theory—which attributed disease to "bad air"—by proposing that cholera was transmitted through contaminated water.12

To investigate the outbreak in the Soho district, Snow utilized skilled reasoning, graphs, and maps to document the location of cholera deaths. He observed that a large number of fatalities were clustered around a specific public water hand pump on Broad Street.21

Snow presented his findings to local authorities, suggesting that the pump was the source of the contagion. Although the officials were initially skeptical, they removed the pump's handle the following day. While the number of new cases subsequently dwindled, the outbreak was already in decline, making it difficult to definitively attribute the end of the epidemic solely to the removal of the handle. Snow’s innovative use of data and mapping to identify the source of the disease remains a foundational contribution to the field of epidemiology.12