How did John Snow trace cholera to a Soho water pump?

Snow plotted the addresses of people who died in the Soho outbreak of August 1854 on a map and saw that the deaths clustered around the water pump on Broad Street. He then checked death records and interviewed residents to learn which water people had actually drunk, and presented the result to local authorities, who removed the pump handle on 8 September 1854. [1] [2]

The outbreak began in Soho on 31 August 1854, and about 500 people died in ten days. Snow used the addresses of the dead to mark each death on a map. The marks gathered thickly around the Broad Street pump, now on Broadwick Street, which residents used for drinking, cooking and washing. Snow also reviewed death records and interviewed household members, and found that most of the dead had lived near the pump and drunk its water. [1] [2]

A map alone could not settle the question. As one account notes, it could equally be used to argue that a foul smell from the pump caused the disease, as miasma theory held. Snow had to show that the water was swallowed. [3]

Snow looked at cases that did not fit the geography. Some victims who lived farther away had obtained water from the Broad Street pump. Most of those he checked were schoolchildren or workers who passed the pump daily. In one case, a woman who had moved away had Broad Street water delivered, and she and her visiting niece died. Some nearby households were spared: they had pumps on their own premises, or, in the case of an alehouse, workers who rarely drank water because they received a daily allotment of beer. [3] [4]

Snow's conclusion did not come from one outbreak. During the 1848-49 epidemic he compared deaths among customers of different London water companies, including supplies drawn from sewage-affected parts of the Thames. His findings had often been ignored or dismissed. He published them in On the Mode of Communication of Cholera and expanded the book in a second edition in 1855. [4] [3]

The popular story that removing the handle ended the epidemic is overstated. One source says new cases had already begun to decline, so the removal should not be seen as the single action that ended it, though the investigation still made a strong case linking contaminated water to cholera. The CDC, by contrast, describes removal as preventing further deaths. The handle was reportedly replaced after the epidemic, and many in the medical world did not favor Snow's theory. [4] [2] [3]

The local priest Henry Whitehead, initially a doubter, later traced the earliest known case to an infant whose soiled linens were emptied into a cesspit beside the pump. The cesspit was cracked and leaked into the well. Authorities later concluded that a leaking sewer or cesspit had contaminated the pump. The cause of cholera remained debated until Vibrio cholerae was isolated in 1883. [3] [1] [2]