A hand-drawn map of deaths ended a cholera outbreak

During London's 1854 cholera epidemic, physician John Snow plotted every death on a street map and found they clustered around a single water pump on Broad Street. He convinced officials to remove the pump handle, and cases collapsed, undermining the reigning theory that disease spread through bad air and founding modern epidemiology.

— John Snow, On the Mode of Communication of Cholera — 2nd edition, 1855