Wednesday 30 May 2012

John Snow's Cholera Map.

John Snow was a nineteenth century London doctor, who is widely credited with the discovery of the transmission mechanism of Cholera, a severe and often fatal infection of the small intestine caused by the bacterium Vibrio cholerae. In Snow's time our understanding of microbiology was in its infancy, and Snow may not even have heard of the concepts of germs or bacteria, but he did, nevertheless, come up with the theory that Cholera was spread by a poison that was formed within the body of its victims and which then went on to infect other people. Prior to this the assumption had been that Cholera, like other diseases, was spread by 'bad air'.

Portrait of John Snow. National Library of Medicine.

A year after he published this theory a major Cholera epidemic hit London (sadly a common occurrence in the mid-nineteenth century), giving him the opportunity to put his theory to the test. Snow set about mapping the course of the outbreak, noting where people were infected, and where they got their water from (most people in London did not have domestic water supplies at the time, instead being dependent on public stand-pipes). Using this method Snow discovered that almost all of the victims got their water from a single company, the Southwark and Vauxhall Waterworks Company, who obtained there water from the lower reaches of the Thames, i.e. water that had passed through the city and had the opportunity to become infected. In particular Snow found that one pump, on the corner of Broad Street (since renamed Broadwick Street) and Cambridge Street, was at the center of a cluster of infections that had killed over 500 people in 10 days. Snows findings led to the removal of the handle of the Broad Street Pump, which reputedly still resides in a local pub, The John Snow, and to a major reworking of London's water supply.

John Snow's map was based upon the geological maps of Robert Mylne, and was eventually presented to the Geological Society of London by the pioneering hydrogeologist William Whitaker. It is featured in the June 2012 edition of Geoscientist, the magazine of the Society and copies if the map are available for purchase from the Society, priced £25 + VAT and postage for fellows of the Society and £35 + VAT and postage for non-fellows.

John Snow's Map of London.

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