He was admitted as a member of the Royal College of Surgeons of England in 1838, graduated from the University of London in December 1844 and was admitted to the Royal College of Physicians in 1850. In 1850 he was also one of the founding members of the Epidemiological Society of London, formed in response to the cholera outbreak of 1849.
Snow was a skeptic of the then-dominant miasma theory that stated that diseases such as cholera were caused by pollution or a noxious form of "bad air". The germ theory of disease had not yet been developed, so Snow did not understand the mechanism by which the disease was transmitted. He first publicised his theory in an 1849 essay, On the Mode of Communication of Cholera, followed by a more detailed treatise in 1855 incorporating the results of his investigation of the role of the water supply in the Soho epidemic of 1854.
Snow later used a dot map to illustrate the cluster of cholera cases around the pump. He also used statistics to illustrate the connection between the quality of the water source and cholera cases. He showed that the Southwark and Vauxhall Waterworks Company was taking water from sewage-polluted sections of the Thames and delivering the water to homes, leading to an increased incidence of cholera. Snow's study was a major event in the history of public health and geography. It is regarded as the founding event of the science of epidemiology.
In June 1858 Snow suffered a stroke while working in his London office. He never recovered, dying on June 16th. He was buried in Brompton Cemetery.
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