John Snow (1813–1858) was a London physician and a founding father of modern epidemiology. He was a pioneer anesthetist who invented a new kind of mask to administer chloroform, which he used on Queen Victoria to assist at the births of her two youngest children. He was an astute clinician and kept meticulously detailed notes about his patients and their diseases. His work on cholera was of lasting value because it demonstrated several fundamental intellectual steps that must be part of every epidemiologic investigation. He began with a logical analysis of the then available facts, which demonstrated that cholera could not be due to a "miasma," a theory that was then popular. It could only be caused, Snow determined, by a transmissible agent, most probably in drinking water.
Having arrived at this logical conclusion, Snow conducted two epoch-making epidemiological investigations in the great cholera epidemic of 1853 to 1854. One was a study of a severe, localized epidemic in Soho, using analysis of descriptive epidemiologic data and spot maps to demonstrate that the cause was polluted water from a pump in Broad Street. His investigation of the more widespread epidemic in South London involved him in an inquiry into the source of drinking water used in some seven hundred households. Snow compared the water source in houses where cholera had occurred with that in houses where it had not. His analysis showed beyond doubt that the cause of the epidemic was water that was being supplied to houses by the Southwark and Vauxhall water company, which drew its water from the Thames downriver, from London, where many effluent discharges polluted the water. Snow found that very few cases occurred in households supplied with water by the Lambeth company, which collected water upstream from London, where there was little or no pollution. Snow's work was remarkable in that it was completed thirty years before Robert Koch identified the cholera bacillus. Snow published his work in a monograph, On the Mode of Communication of Cholera (1855). This classic book has been reprinted in several modern editions and is still used as a teaching text in courses of epidemiology.