John Snow (1813–1858; not to be confused with the modern-day TV character Jon Snow) was a London physician. He helped to control a mass outbreak of cholera in the Soho district of the City of Westminster, London, England, in 1854. He deduced that the disease was being spread through the borough’s public wells. By administering doses of chlorine to the offending wells, he brought the epidemic to a standstill.
The addition of chlorine to public water supplies became commonplace in the beginning of the twentieth century, bringing about dramatic reductions in outbreaks of waterborne diseases such as typhoid and cholera.