John Snow

In the 1800s there were large epidemics of cholera in Europe and America that killed thousands of people. John Snow (shown above) was a physician in London who spent several decades studying cholera in a systematic way. He is most often credited with solving an outbreak of cholera that occurred in London in 1854 (the outbreak is described below in the map of Soho London), but his studies of cholera were much more extensive than that. The first cholera epidemic in London struck in 1831 when Snow was still an apprentice. Another large epidemic occurred in 1848 and lasted through 1849.

The prevailing opinion was that cholera was spread either by miasmas or by person-to-person contact, Snow began examining the victims and found that their initial symptoms were always related to the gastrointestinal tract. Snow reasoned that, if cholera was spread by bad air, it should cause pulmonary symptoms, but since the symptoms were gastrointestinal, perhaps it was transmitted by water or food consumption. In fact, cholera is caused by the bacterium, Vibrio cholera, which is transmitted by the fecal-oral route, that is by ingestion of water or food that is contaminated with sewage.
In August 1849 Snow published a paper entitled "On the Mode of Communication of Cholera" in which he presented his theory that the disease was acquired by ingestion of contaminated water, but his theory did not get much traction with the medical establishment. The epidemic ended in 1849, but Snow continued to collect data on the pattern of disease and began finding evidence that linked cholera to specific sources of water. Many Londoners received their water from hand pump wells that were located throughout the city.

In retrospect, Snow made several important contributions to the development of epidemiologic thinking:

• He proposed a new hypothesis for how cholera was transmitted.
• He tested this hypothesis systematically by making comparisons between groups of people.
• He provided evidence for an association between drinking from the Broad St. well and getting cholera
• He argued for an intervention that prevented additional cases (removal of the pump handle).