Cholera is caused by the bacterium Vibrio cholerae which is spread through contaminated water and food or direct contact with an infected person. It causes severe diarrhea and dehydration which can be life-threatening if untreated. Cholera remains a global threat especially in developing countries with inadequate sanitation and clean water supplies where outbreaks still occur. Understanding how cholera spreads helped the physician John Snow trace a 1854 London outbreak to a contaminated water pump, pioneering epidemiology.