In 1854, cholera killed nearly 500 people in one London neighborhood in just ten days.
Dr. John Snow reached for an unlikely tool: a map, marking every case. The dots clustered around a single public water pump on Broad Street.
While others blamed bad air, Snow's map pointed at the water. Officials pried off the pump handle, though the outbreak was already easing.
He was right decades before anyone could see a germ — because he trusted a map of the dead.