John Snow, Cholera and the Battle for Broad Street

Densely packed, London's Broad Street in 1854 was a perfect breeding ground for a cholera outbreak
Densely packed, London's Broad Street in 1854 was a perfect breeding ground for a cholera outbreak

by Ben Gaskin


August 31, 1854 — In the 1854 Broad Street cholera outbreak in London, John Snow made his name as one of the founders of modern epidemiology. Previously, cholera had been thought to be caused by particles called “miasmata” that emanated from decomposing matter and other such unclean sources. Snow, however, was skeptical of this theory. He believed instead in a germ theory of cholera: that it was caused by an unidentified germ, which Snow theorised was transmitted via drinking water.

Snow investigated this theory, and the outbreak, by focusing on the water sources of London residents and the rates of cholera in the areas served by them. He found, most notably, “that the water of the companies of the Surrey Side of London … is by far the worst of all those who take their supply from the Thames.” Other companies had cleaner water supplies—not only did they draw from better sources but also better filtered what water they drew.

The peculiarities of water supplies at that time also allowed Snow to unknowingly conduct an almost perfect double-blind experiment:

In many cases a single house has a supply different from that on either side. Each company supplies both rich and poor, both large houses and small; there is no difference in the condition or occupation of the persons receiving the water of the different companies ... As there is no difference whatever either in the houses or the people receiving the supply of the two Water Companies, or in any of the physical conditions with which they are surrounded, it is obvious that no experiment could have been devised which would more thoroughly test the effect of water supply on the progress of Cholera than this, which circumstances placed ready made before the observer. The experiment too, was on the grandest scale. No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station, from gentlefolks down to the very poor, were divided into two groups without their choice, and, in most cases, without their knowledge; one group being supplied water containing the sewage of London, and amongst it, whatever might have come from the cholera patients, the other group having water quite free from such impurity.

– John Snow, 1855: On the mode of communication of cholera

This lead Snow to study the purity of water in samples drawn from each household source. Ultimately, he attributed the outbreak of cholera to impure water sources. This finding was borne out when he had prisons switch their water sources: within days of acquiring a better source, the rate of new cholera cases declined substantially.

Snow’s methods in the Broad Street cholera outbreak laid the groundwork for what would become modern epidemiology. He was also influential in the widespread adoption of anaesthesia, as well as various improvements to public waste and water systems. Altogether Snow had an undeniably impressive effect on global health and his influence can be seen throughout the modern world.

Published: April 13, 2020

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