Intro: Dr. John Snow's Map of the 1854 London Cholera Outbreak GIS Analyses of the Cholera Data

Dr. John Snow is regarded as one of the founding father of modern epidemiology.  London suffered a series of cholera outbreaks during the mid-19th century, and Snow theorized that cholera reproduced in the human body and was spread through contaminated water.  The conventional wisdom at the time was that diseases were spread by "miasma" in the air.

London's water supply system consisted of shallow public wells where people could pump their own water to carry home, and about a dozen water utilities that drew water from the Thames to supply a jumble of water lines to more upscale houses. London's sewage system was even more ad hoc: privies emptied into cesspools or cellars more often than directly into sewer pipes. The pervasive stench of animal and human feces combined with rotting garbage made the miasma theory of disease seem very plausible: Disease was more prevalent in lower-class neighborhoods because they stank more, and because the moral depravity of the lower classes made them more vulnerable to disease.

The September 1854 cholera outbreak was centered in the Soho district, close to Snow's house. Snow mapped the 13 public wells and all the known cholera deaths around Soho, and noted the spatial clustering of cases around one particular water pump on Broad Street.  He examined water samples from various wells under a microscope, and confirmed the presence of an unknown bacterium in the Broad Street samples. Despite strong scepticism from the local authorities, he removed the handle from the Broad Street pump and halted the outbreak. 

Snow subsequently published his map with a discussion of his analysis of it. An image of a medium-resolution downloadable scan of Snow's map available from Wikipedia is shown below.  This map shows the locations of the 13 public water sources in and around Soho, and shows cholera deaths by home address, marked as black bars stacked perpendicular to the streets. 

Although the large workhouse just north of Broad Street housed over 500 paupers, it suffered very few cholera deaths because it had its own well (not shown on the map).  Likewise, The workers at the brewery one block east of the Broad Street were allowed all the beer they could drink on the job, and didn't use the well, and none of them contracted cholera. Many of the deaths further away from the Broad Street pump were people who walked to work or market on the Broad Street and drank from that well. Other deaths were attributable to the superior taste of the water from Broad Street, particularly compared to the smelly water from the Carnaby Street/Little Marlborough Street well.

Analysis of Dr. Snow's map