John Snow

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John Snow (1813-1858) played a crucial role in the development of epidemiology through his discoveries about the cause of cholera outbreaks. He began working on this during the first cholera epidemic in Britain, in 1831, and continued to study the disease until his death before the final pandemic in 1866. An inspiring story of rising to success, Snow came from a farming family and, at the age of fourteen, moved to Southern London from York (on foot!) to work under William Hardcastle, a great surgeon and teacher. After years of apprenticeship and the development of his own successful practice, he became a highly regarded physician with many references and commendations to his name. While his specialty was obstetrics, he cared for other patients and experimented with various medicines.

L0058154 Snow-type chloroform inhaler, London, England, 1848-1870
Snow-type chloroform inhaler, London, England, 1848-1870 Credit: Science Museum, London. Wellcome Images.

One of the most famous of his early experiments includes his ventures into anesthetics using ether gases. He studied the various methods of delivery as well as dosage amounts and side effects, never straying from the possible conflicts of self-testing when no test subjects were available and willing. He was so successful, in fact, that he assisted Queen Victoria through her last two childbirths, gaining her support as a notable physician and scientist in the last decade of his life. His familiarity with the property of gases  helped to form his speculations on the transmission of cholera in London.

One of the most commonly accepted theories for cholera transmission was that miasmas (bad air caused by pollution on the ground, in the water, or in the air) caused people to become ill. This was a particularly successful theory because the Thames River which supplied southern London with drinking water was also the recipient of the city’s waste. This pollution of the water with human excrement among other debris caused the river to become putrid in appearance, smell, and taste (so much so, in fact, that at one point it was called “The Great Stink of London”, spurring legislation towards sewage systems and waste treatment). As cholera often came in warmer months like August, the smells emanating from the Thames were at their worst at the same time that cholera began to run rampant through the city streets. People were unsure of how it moved from person to person and home to home, finding answers in miasmas which often led to mass exoduses from the city. While the miasmic theory seemed to explain many aspects of cholera epidemics, there phenomena that it did not account for. John Snow saw one of these instances in the fact that doctors very rarely became ill from treating cholera patients despite breathing the “diseased” air around them. Troubled by these inconsistencies, and seeking to apply his knowledge of gases to the understanding of cholera epidemics, he began to experiment and explore other modes of transmission. He acknowledged that something was causing person to person transmission, but noted that those who were simply in the room with the ill did not get sick all the time. Further research into his own patients and the case reports of other doctors proved that the air around the patients and the damaged blood of them (the severe and rapid dehydration from cholera caused blood to become thick and tar-like) were not the principle causes of the disease. This left him with the alimentary canal (digestive tract) as the receptacle for the disease, therefore limiting transmission to the same as well.

Over time and with the use various studies and experiments he was able to narrow the mode of human to human transmission to that of the fecal-oral route which occurred when an individual did not clean their hands properly after dealing with a sick individual. This accounted for their doctors not becoming ill. With this notion in mind, he looked to find ways in which contaminated fecal matter was getting into the water supply of London homes which were affected during the rampant spread of the disease during the infamous outbreaks. It took some time as he had to wait for the disease to be wreaking havoc in the city, but eventually found evidence supporting his idea that contaminated fecal matter was entering the water supply and spreading the disease. Disputes followed his original findings, a small pamphlet published on his discoveries, and the miasmic theory continued to hold power over Snow and other discoveries. It was not until the next year, with the famous Broad Street pump study that Snow was able to not only prove his theory of person to person transmission, but the origin of the disease residing in the fecal contaminated water of the well-pump.

L0028567 Broad Street (latterly Broadwick Street), Soho, with a white
Broad Street (latterly Broadwick Street), Soho, with a white silhouette replica of the Broad Street pump identified by John Snow as a source of cholera-infected water. Credit: Wellcome Library, London.

As families became sick and the disease began to spread, Snow accessed death records and, upon analysis, found that these particular individuals all frequented the Broad Street pump. While this helped support his theory that the disease was transmitted through water to the alimentary canal, it did not explain how the disease got there in the first place. Further questioning and deductive skills allowed him to find a child who had suffered from diarrhea two days prior to the outbreak. This child lived the closest to the pump and it was found that the septic system of the house in which the child lived had leaked sewage through the old and rotted brickwork into the well which was less than three feet away, initiating the epidemic. Further analysis of the water proved that the disease was there as examinations observed white flecks within, warranting the term “rice water” that was commonly associated with the disease. He presented his findings to the commissioner of health and the very next day, the handle from the pump was removed.

L0063431 Map showing deaths from Cholera in Broad Street...
Map showing deaths from Cholera in Broad Street, Golden Square and the neighbourhood, from 19th August to 30th September 1854 1855 On the mode of communication of cholera / John Snow Published: 1855. Credit: Wellcome Library, London.

Snow was successful in not only identifying the mode of communication from person to person, but the source of the initial outbreaks before his death in 1858. The bacterium that causes cholera would be discovered later in the century by Robert Koch, but even without this knowledge, Snow proved essential in the process of making London a cleaner and safer place to live.

Written by Brooke Shaw


Anesthesia and the Queen. (UCLA Department of Epidemiology: Public Health.) Accessed April 12, 2016.

Banerjee, Jacqueline. “John Snow and Waterborne Diseases.” Victorian Web. July 8, 2012. Accessed March 28, 2016.

John Snow and the Broad Street Pump. (UCLA Department of Epidemiology: Public Health. 2003). Accessed April 12, 2016.

Snow, John. Snow on Cholera: Being a Reprint of Two Papers. (New York: The Commonwealth Fund; London: H. Milford, Oxford University Press, 1936). Accessed March 28, 2016.

Snow, Stephanie J. “Commentary: Sutherland, Snow and water: the transmission of cholera in the nineteenth century.” International Journal of Epidemiology. (2002). Accessed March 31, 2016.Banerjee, Jacqueline.