John Snow & the Broad Street Cholera Outbreak

John Snow

Dr John Snow was an English physician, credited as “the father of epidemiology”, he was also a leader in the development of anaesthesia.  He was born in 1813 and moved to London in 1837.

Cholera in London

Since 1831 London had been suffering a series of Cholera outbreaks, which killed a total of 14,137 people.

This was now 1854 and John Snow was trying to determine the cause of the latest “Golden Square” cholera outbreak. He was skeptical of the miasma theory which had been dominant since around the 2nd century.  According to Miasma theory, diseases like cholera and bubonic plague are spread by a noxious form of “bad air”. Louis Pasteur wouldn’t start his germ theory experiments until the 1860’s, so Snow was ahead of the curve.

Snow’s Epidemiology

Snow spoke with local residents about their habits. This lead him to identify the source of the outbreak to the public water pump on Broad Street.  His chemical analysis & microscopy of water from the Broad Street pump didn’t conclusively prove its danger. However, his studies of the pattern of the outbreak convinced the local council to remove the pump’s handle. 

This action was credited as ending the outbreak, but Snow suspected that the epidemic was already in rapid decline. He wrote:

“There is no doubt that the mortality was much diminished, as I said before, by the flight of the population, which commenced soon after the outbreak; but the attacks had so far diminished before the use of the water was stopped, that it is impossible to decide whether the well still contained the cholera poison in an active state, or whether, from some cause, the water had become free from it.”

J. Snow, On the Mode of Communication of Cholera. London: John Churchill, 1855. Accessed: Aug. 31, 2022. [Online]. Available:

Snow later used this dot map to illustrate the cluster of cholera cases around the pump:

John Snow's dot map of the 1854 Broad Street Cholera Epidemic

A map taken from a report by Dr. John Snow.  Wellcome Collection. Public Domain Mark.

Snow used statistics to illustrate the connection between the quality of the water source and cholera cases. This showed that homes supplied by the Southwark and Vauxhall Waterworks Company had a cholera rate fourteen times that of those supplied by Lambeth Waterworks Company.  Southwark and Vauxhall were taking water from sections of the Thames that were more polluted by sewage than Seething Wells upstream, where Lambeth Waterworks Company were drawing their water.

Snow wrote this in a letter to the editor of the Medical Times and Gazette:

“On proceeding to the spot, I found that nearly all the deaths had taken place within a short distance of the pump. There were only ten deaths in houses situated decidedly nearer to another street-pump. In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street, as they preferred the water to that of the pumps which were nearer. In three other cases, the deceased were children who went to school near the pump in Broad Street.
With regard to the deaths occurring in the locality belonging to the pump, there were 61 instances in which I was informed that the deceased persons used to drink the pump water from Broad Street, either constantly or occasionally.
The result of the inquiry, then, is, that there has been no particular outbreak or prevalence of cholera in this part of London except among the persons who were in the habit of drinking the water of the above-mentioned pump well.
I had an interview with the Board of Guardians of St James’s parish, on the evening of the 7th and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the following day.”

J. Snow, ‘Snow, John. Med. Times and Gazette, n. s. vol. 9, Sept. 23, 1854, pp. 321-322.’ (accessed Aug. 31, 2022).

Unfortunately after the epidemic subsided, the government rejected Snow’s theory as the fecal-oral route of disease transmission, was too unpleasant to contemplate.  So they replaced the pump handle.

Later work

William Farr had been one of Snow’s biggest opponents on the subject. In 1866 he finally realised the validity of Snow’s diagnosis, while investigating an outbreak of cholera in Bromley by Bow. Farr issued what may have been one of the first boil water notices. This was 8 years after Snow’s death. Farr still denied Snow’s explanation, but accepted that water had a role in the spread of the illness. 

Researchers later found that the broad street pump’s well had been dug less than one meter from an old cesspit. And the cesspit was leaking faecal bacteria. The nappy of a baby, who caught cholera somewhere else, had been washed into this cesspit. 

John Snow: Further Reading