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kottke.org posts about John Snow

The 1850s Map that Made Modern Epidemiology

posted by Jason Kottke   Jun 18, 2020

In 1854, Dr. John Snow produced a map of a London cholera outbreak which showed deaths from the disease concentrated around a water pump on Broad Street. The prevailing view at the time was that cholera spread through dirty air, but Snow hypothesized that it was actually spread through water and constructed this early medical data visualization to help prove it.

John Snow Cholera Map

Through a mix of personal interviews, clever detective work, and data analysis that included tables and a famous map, Snow managed to stop the outbreak and convince local public health officials, eventually, that cholera could be transmitted through water, not a miasma. Since his breakthrough study, the map has become an iconic piece of epidemiological history, as an illustration of keen detective work, analysis, and visual representation with a map that, even today, tells a story.

Aside from the cluster of deaths around the pump (which could be argued were the result of a miasma cloud and not contaminated water), stories of nearby people who didn’t get sick (brewers who drank the beer they produced rather than well water, people in buildings with their own wells) and far away people who died because they had drunk water from the well were also essential in proving his theory:

I was informed by this lady’s son that she had not been in the neighbourhood of Broad Street for many months. A cart went from broad Street to West End every day and it was the custom to take out a large bottle of the water from the pump in Broad Street, as she preferred it. The water was taken on Thursday 31st August., and she drank of it in the evening, and also on Friday. She was seized with cholera on the evening of the latter day, and died on Saturday

You can read more about John Snow and how his map changed science and medicine in Steven Johnson’s excellent Ghost Map.