It all starts with a pump. In this episode of Vox Almanac, Vox’s Phil Edwards explores Dr. John Snow’s map of the Broad Street Pump, which changed epidemiology forever. In 1854, news spread about a mysterious new cholera outbreak in London. At the time, doctors and scientists largely believed the disease traveled in a “miasma” — a floating cloud of sickness. Dr. John Snow suspected bad water might actually be the agent of transmission — and he wanted to prove it in time to stop the outbreak. Through a mix of personal interviews, clever detective work, and data analysis that included tables and a famous map, Snow managed to stop the outbreak and convince local public health officials, eventually, that cholera could be transmitted through water, not a miasma. Since his breakthrough study, the map has become an iconic piece of epidemiological history, as an illustration of keen detective work, analysis, and visual representation with a map that, even today, tells a story.