Like most people you’ve probably learned this year what an epidemiologist is – someone who studies population-wide patterns of health and disease – and become accustomed to seeing them quoted in the media. Our Policy Advisor (Child Health) Michelle Imison investigates the fascinating, founding event in the science of epidemiology…

John Snow was a nineteenth-century English physician, born in York. He did his early medical training in nearby Newcastle-upon-Tyne, where he experienced a cholera outbreak and treated many people suffering from the effects of this disease.

He later moved to London to complete his studies, and set up his practice in Soho – in the centre of what is now the West End theatre district. Snow became a specialist in obstetric anaesthia: he personally administered chloroform to Queen Victoria when she gave birth to the last two of her nine children, helping to create a widespread public acceptance of anaesthetic use in childbirth. 

Then on 31 August 1854, after several outbreaks elsewhere in London, there was a major outbreak of cholera in Soho. Snow later called it ‘the most terrible outbreak of cholera which ever occurred in this kingdom’. Over the next three days 127 people on or near Broad Street, in the centre of Soho, died. By 10 September, 500 people had died and the mortality rate was 12.8 percent in some parts of the city. At the end of the outbreak, there were 616 dead.

Above: a commemorative replica of the Broad St pump; the triangular piece at the far right of the picture is where the handle would have been. The pub in the background is called the ‘John Snow’.

The dominant theory at the time was that diseases like cholera were caused by pollution or ‘bad air’. Although germ theory hadn’t yet been developed and Snow wasn’t certain how the disease was transmitted, his previous investigations of cholera made him sceptical of this idea of foul air.

Instead he and the local reverend went out and talked to the residents, and Snow identified the source of the outbreak as the public water-pump on Broad Street: the greatest concentration of cases was in the part of the neighbourhood where this pump was the closest available water-source. His study of local patterns of disease convinced him that this was the case, even though he could not conclusively demonstrate it from chemical and microscope examination. (Sure enough, researchers later discovered that this well had been dug only three feet from an old cesspit – common at the time under most homes – which had begun to leak.)

In a later letter to the editor of the Medical Times and Gazette Snow reported his findings and the actions that resulted:

‘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 [Broad Street] pump well.

I had an interview with the Board of Guardians of St James’ parish, on the evening of 7 September, and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the following day.’

But after the cholera epidemic had subsided, government officials replaced the pump handle: they had responded to an urgent threat to the population, but rejected Snow’s theory. To accept his proposal would have meant indirectly accepting the faecal-oral route of disease transmission, which was apparently too unpleasant for most of the public to contemplate (although of course it is now well-recognised, particularly in the case of diseases such as polio). 

Above: a plaque near the replica of the water pump.

It wasn’t until 1866 that William Farr – one of Snow’s colleagues and chief intellectual opponents – realised the validity of his thesis when investigating another outbreak of cholera, again in London, and issued immediate orders that unboiled water was not to be drunk.

At the age of 45 John Snow suffered a stroke from which he died shortly thereafter, on 16 June 1858. He sadly never lived to see his ideas widely accepted, but his work on cholera had a huge influence on improvements in sanitation that began later in the nineteenth century and which have been central to reducing burdens of many diseases throughout the world. Dr John Snow is one of those people of whom Sir Isaac Newton might have been speaking when he said that intellectual advances and progress in human life comes from ‘standing on the shoulders of giants’.