Health

In just 10 days over the summer of 1854, 500 people died of cholera in the Soho neighborhood of London. The city's population had more than doubled to 2.3 million people in the first half of the 1800s, and its sewage system could not keep up. But the streams of human waste flowing into the street and seeping into the water supply were considered unconnected to the cholera crisis. The prevailing theory of the day was that bad air — miasma — caused illness.

The English physician John Snow thought differently. Five years before the outbreak he had suggested that the diarrheal disease was actually caused by a waterborne infection rather than miasma. He soon had a chance to test his theory, mapping the location of cholera-related deaths in Soho. Snow realized that the victims used one specific water pump on Broad Street, and he persuaded city officials to remove the pump's handle to prevent anyone else from using it. With the source eliminated, the outbreak, which had already passed its peak, ended in days.