Snow first became interested in cholera at Newcastle-on-Tyne during the epidemic of 1831-1832, and recurrent outbreaks of the disease gave him the opportunity to investigate it in detail. His paper on cholera, published shortly after his (extremely rare) 31-page pamphlet On the Mode of Communication of Cholera (1849) contained his first demonstration of the specific nature of the disease, which he defined correctly as an infection of the alimentary canal transmitted by ingesting fecal matter from cholera patients, in most cases via contaminated water. Snow proved his theory of cholera transmission by collecting data on a large number of outbreaks and correlating them to local water supplies. He argued, based on his data, that cholera was caused by “a specific living, waterborne, self-reproducing cell or germ” (Dictionary of Scientific Biography)—a conclusion all the more remarkable in that it predated the germ theory of disease by over a decade.
Snow may have been motivated to contribute his paper to the London Medical Gazette because a review of his separately published pamphlet published in that journal on pp. 466-470 of the 1849 volume stated that he had not proved the contagious nature of cholera.