Sir John Snow's cholera map from 1854 was one of the earliest examples of data visualization and a key moment in the history of epidemiology. By mapping cases of cholera in London onto a street map, Snow was able to see that many cases were clustered around the Broad Street water pump, allowing him to deduce that contaminated water was spreading cholera. His findings helped establish germ theory and led to improvements in urban sanitation that reduced disease outbreaks.