An Anaesthetist Beat London's Cholera With a Map and a Wrench

John Snow was, by day, the doctor who anaesthetised Queen Victoria during the birth of two of her children. By evening he was investigating cholera. His suspicion, against the dominant 19th-century theory of miasma — bad air rising from filth — was that the disease moved through contaminated water.

In late August 1854 a violent cholera outbreak hit Soho, a few streets from Snow's home in Sackville Street. Within a week several hundred people in a small district were dead. Snow walked the neighbourhood, knocked on doors, and recorded the address of each death. He plotted them on a street map as black bars, each bar a victim. The bars stacked into a clear cluster around a single public water pump on Broad Street, now Broadwick Street.

The outliers were the most useful part of the map. A nearby workhouse, with its own well, had no deaths. The Lion Brewery on the same street, whose workers drank malt liquor, had none. A widow who had moved to Hampstead but liked the taste of Broad Street water and had it carted to her died of cholera.

On 8 September 1854 Snow presented his evidence to the Board of Guardians of St James's Parish. They authorised the removal of the pump's handle. The outbreak, already declining for other reasons, ended within days. Snow was honest in his later writings that the timing made causal interpretation hard.

The contemporary medical establishment was not convinced. The Vibrio cholerae bacterium would not be properly identified until Robert Koch isolated it in 1883. What Snow had was a piece of public-health methodology — careful spatial analysis of an outbreak — that the field has used ever since.