A Case Study in Public Health

The following is an excerpt from Pan American Health Organization’s “Public Health: What’s it really about?”

It was an English physician of the mid-19th century who made one of the greatest contributions to public health. John Snow (1813-1858), a pioneering anesthesiologist and epidemiologist distinguished himself through his creativity and good use of scientific data. During a cholera outbreak that ravaged London in 1831-32, Snow began to investigate the cause and means of transmission of the disease. In 1849, he published a pamphlet suggesting that cholera was a contagious disease caused by a toxin that reproduces in the human body and which is found in the vomit and feces of those infected. He believed that the principal, though not the only, means of transmission was water polluted with the toxin. At the time, it was believed that diseases were transmitted through inhalation of the "miasma," or bad air, and Snow's hypothesis was not widely accepted.

However, in 1854, Snow would prove his theory during the course of another epidemic in London, when he documented cases of cholera and compared the disease's incidence among the clients of two different companies that supplied the city's water. He showed that cholera was much more frequent among clients of the Southwark and Vauxhall Water Company, which drew its water from the lower Thames, which was contaminated with London sewage. Clients of the Lambeth Waterworks Company, which drew its water upriver, suffered a much lower incidence.

The evidence supported his theory, but an incident that has become legend finally convinced the unbelievers. In the neighborhood surrounding the intersection of Cambridge and Broad streets, a concentration of cholera cases produced 500 deaths in 10 days. Upon investigation, Snow concluded that the problem lay in the Broad Street water pump, and he suggested that officials remove the handle from the pump, so that residents could not consume the polluted water. They did, and the epidemic was halted.

The incident went down in history as an example of effective epidemiology and as one of the first applications of public health principles for disease prevention and control. (With public health advances, cholera eventually disappeared from the Americas until 1991, when a new epidemic swept the countries but was controlled relatively quickly through intense public health efforts.)

Snow's 1854 map of London (below) plots cholera deaths with dots and 11 area water pumps with crosses. From E.W. Gilbert, “Pioneer Maps of Health and Disease in England,” Geographical Journal, 124 (1958), 172-183.

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