John Snow’s “Grand Experiment,” 1855

As London suffered one of its worst cholera outbreaks, in the summer of 1853, John Snow took it upon himself to prove that the disease was transmitted through drinking water.

Aug 1, 2010
Daniel Grushkin


For 14 weeks in the late summer of 1853, London suffered one of its worst cholera outbreaks. The leading voices in medicine believed the disease emanated from the foul gasses of London’s polluted streets. John Snow, a prominent anesthesiologist, was convinced otherwise. “He was the original intellectual maverick,” says Ralph Frerichs, professor emeritus of epidemiology at the UCLA School of Public Health. With this map of South London, Snow believed he could prove that cholera was transmitted through drinking water. But while he would later become known as the father of modern epidemiology, his peers of the time rejected his theory. Now, some contemporary medical historians are agreeing that Snow may not have had an ironclad case.

Map courtesy of The John Snow Site (
#1 - The colors represent the territories of two companies that piped drinking water to South London. The Lambeth Company (red) drew its water from upstream in a relatively clean area of the Thames. The Southwark and Vauxhaul Company (blue, now faded to green) had its intakes in the polluted waters downstream at Battersea Park. Snow suspected that Southwark was piping cholera into people’s homes.
#2 - This purple (now faded to brown) area came to be known as “the grand experiment.” It produced the perfect condition for an environmental study. The pipes in the area were so intermingled that Snow had a near random sampling of neighbors, virtually identical in every way except for their source of water. To show that the disease was carried through the tap, “all that [I] required was to learn the supply of water to each individual house where a fatal attack of cholera might occur,” Snow wrote in his book On the Mode of Communication of Cholera, where the map was originally published in 1855.
Snow estimated 315 deaths per 10,000 houses for Southwark compared to only 37 for Lambeth—a seemingly closed case. But Snow based his numbers on a parliamentary report that didn’t have house-to-house resolution he required. Even Snow himself later admitted that he didn’t have that data to fully show the relationship between cholera and water supply. As such, his peers went unconvinced. “The grand experiment promised in the map was a failure,” says Tom Koch, author of the forthcoming Disease Maps: Epidemics on the Ground (where the map will be featured).