How did john snow use maps to study the spread of cholera?

John Snow used maps to study the spread of cholera during the 1854 outbreak in London by plotting the locations of cholera cases and public water pumps on a detailed map of the area. By visually correlating the incidence of cholera with proximity to the Broad Street pump, he identified a cluster of cases linked to that specific water source. This mapping technique provided compelling evidence that contaminated water was the source of the outbreak, leading to significant public health interventions. Snow's work laid the groundwork for modern epidemiology and the use of geographic data in health studies.