Map-making and myth-making in Broad Street: 

the London cholera epidemic, 1854

Source: The Lancet 356: 64 - 68, 2000.
Howard Brody, Michael Russell Rip, Peter Vinten-Johansen, Nigel Paneth, Stephen Rachman

Click words in text to see location in 1859 map of London. 

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Introduction

The outbreak of cholera in the vicinity of Golden Square, central London, in the late summer of 1854, and the subsequent removal of the handle from the Broad Street pump, have become an enduring feature of the folklore of public health and epidemiology. To fully understand the incident requires an accurate reconstruction of the role of Dr John Snow, who proposed that cholera was commonly transmitted by drinking water [1,2]. Modern writers persist in disseminating not the facts but an apocryphal story to support a desired conclusion, as in this representative example:

"[Snow] sat down one afternoon with a map of London, where a recent outbreak had killed more than 500 people in one dreadful 10-day period.

He marked the locations of the homes of those who had died. From the marks on his map, Snow could see that the deaths had all occurred in the so-called Golden Square area. The most striking difference between this district and the rest of London was the source of its drinking water. The private water company supplying the Golden Square neighborhood ... was getting its water from a section of the Thames River that was known to be especially polluted.

So Snow went down to Broad Street, where he suspected that one particular pump was the source of the contaminated water. And, in a gesture that still reverberates among public health scholars today, he removed the handle of the Broad Street pump."

Once the pump was out of commission, the epidemic abated.[3]

This version of events states that Snow constructed a spot map to arrive at the correct answer; it alleges that he proceeded in an orderly manner from facts (the locations of deaths) to hypothesis (the infectivity of the water), and it assumes that any reasonable person, looking at such a spot map, would have drawn the same conclusion.

We will consider three sets of maps related to the Golden Square cholera outbreak, and will show that other observers looked at even more detailed and accurate maps than Snow's, yet came to different conclusions about the cause of the cholera outbreak. Moreover, Snow developed and tested his hypothesis will before he drew his map. The map did not give rise to the insight, but rather it tended to confirm theories already held by the various investigators.