How did John Snow connect a London cholera outbreak to the Broad Street water pump?

During the 1854 cholera outbreak in London, physician John Snow challenged the prevailing miasma theory—which attributed disease to "bad air"—by proposing that cholera was a waterborne illness caused by germ-contaminated matter.12

To investigate this, Snow utilized meticulous data collection and mapping techniques in the Soho district. He observed that a high number of cholera deaths were geographically clustered around a specific public water hand pump on Broad Street. By documenting these cases and mapping their proximity to the pump, he identified the water source as the likely vehicle for the disease's transmission.12

Snow presented his findings to local authorities, arguing that the pump was the source of the outbreak. Although officials were initially skeptical, they agreed to remove the pump's handle, effectively disabling it. While the subsequent decline in new cholera cases provided strong circumstantial evidence for his theory, Snow further solidified his conclusions through his "Grand Experiment," which compared mortality rates between populations served by different water companies, demonstrating that those consuming sewage-polluted water suffered significantly higher death rates.12