Cholera is a dangerous bacterial disease that causes severe diarrhea and dehydration. Someone with cholera can lose so much water from their body so quickly that they become dangerously ill within hours. The disease spreads through contaminated water: when sewage mixes with drinking water, the cholera bacteria can infect anyone who drinks it.

For most of human history, cholera killed millions of people in devastating epidemics that swept through cities. In the 1850s, a London doctor named John Snow proved that cholera spread through contaminated water, not “bad air” as people had believed. His detective work during a cholera outbreak helped launch modern epidemiology, the science of tracking and stopping disease spread.

Today, cholera rarely appears in places with modern water treatment and sewage systems. Clean drinking water and proper sanitation can prevent the disease entirely. But cholera still threatens people in areas without clean water infrastructure, especially after natural disasters damage water systems. The disease remains serious but is now treatable: doctors can save cholera patients by quickly replacing their lost fluids, often with a simple mixture of clean water, salt, and sugar.