"A major outbreak of cholera reached the district of Soho, London, in August 1854. This was the third cholera outbreak in London, having previously occurred in 1832 and 1849. In the mid-19th century, Soho had a serious problem with filth due to the large influx of people and a lack of proper sanitary services: the London sewer system had not reached Soho at this point and drainage was poor throughout London. It was common at the time to have a cesspit under most homes.

By talking to local residents (with the help of the Reverend Henry Whitehead), Snow identified the source of the outbreak as the contaminated public water pump on Broad Street (nowBroadwick Street). He did this by mapping the deaths from cholera, and noted that they were mostly people whose nearest access to water was the Broad Street pump (see map below from On the Mode of Communication of Cholera, 2nd ed.). His studies of the pattern of the disease were convincing enough to persuade the local council to disable the well pump by removing its handle. This action has been credited with contributing significantly to the containment of the disease in the area. It was later discovered that the water for the pump was polluted by sewage contaminated with cholera from a nearby cesspit."

However, Snow’s theory was not new in 1854. He had argued earlier that it was not an airborne disease in his published essay, On the Mode of Communication of Cholera, in 1849. The germ theory was not developed at this point, so Snow was unaware of the mechanism by which the disease was transmitted, but evidence led him to deduce in 1854 that it was not due to breathing in foul air. In 1855 a second edition was published, incorporating the results of his investigation of the Soho epidemic of 1854.

Hand pumps like that on Broad Street were not the only source of Londoners’ water, or Snow’s only object of study during the 1854 cholera outbreak. The Lambeth Water Company and the Southwark and Vauxhall Water Company were both supplying mechanically-pumped water to residents of South London (see map below from Tracts 376). Snow recorded cholera attacks in this area, alongside information about the water supply to the houses affected. 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.