During the 1830s and 1840s, when severe cholera epidemics threatened London, Dr. Snow had become interested in the cause and transmission of the disease. In 1849, he published a brief pamphlet, On the Mode of Communication of Cholera, suggesting that cholera is a contagious disease caused by a poison that reproduces in the human body and is found in the vomitus and stools of cholera patients. He believed that the main, although not only, means of transmission was water contaminated with this poison. This differed from a commonly-held theory that diseases were transmitted by inhalation of vapors. The pamphlet caused no great stir, and Dr. Snow's argument was only one of many hopeful theories proposed during a time when cholera was causing great distress.
Dr. Snow was able to prove his theory in 1854, when another severe epidemic of cholera occurred in London. Through painstaking documentation of cholera cases and correlation of the comparative incidence of cholera among subscribers to the cities two water companies, he showed that cholera occurred much more frequently in customers of one water company, the Southwark and Vauxhall. This company drew its water from the lower Thames, where it had become contaminated with London sewage, whereas the other company obtained water from the upper Thames. Dr. Snow's evidence soon gained many converts.
A striking incident during this epidemic has now become legendary. In one particular neighborhood, the intersection of Cambridge Street and Broad Street, the concentration of cholera cases was so great that the number of deaths reached over 500 in ten days. Dr. Snow investigated the situation and concluded that the cause was centered around the Broad Street pump. He advised an incredulous but panicked assembly of officials to have the pump handle removed, and when this was done, the epidemic was contained.
The pump handle has remained a symbol of effective epidemiology, and today the John Snow Pub, located near the site of the former pump in London, boasts of having the "original" handle. A John Snow Society has been formed to honor the memory of the epidemiologist (the only requirement being that one visit the John Snow Pub while in London). Dr. Snow was a skilled practitioner as well as an epidemiologist, and his creative use of the scientific information of his time is an appropriate example for those interested in disease prevention and control.