John Snow and the Broad Street Pump

The dominant theory of epidemic disease in 1854 was miasmatic: foul air rising from decaying organic matter was thought to carry illness into vulnerable lungs. It had motivated public health work, including London's sewer reforms. But it predicted that cholera should track with smell and filth in a diffuse way, which did not match the sudden, localized outbreaks physicians saw. John Snow, a London physician and anesthetist, had argued in an 1849 pamphlet that cholera spread through contaminated water, and he was looking for proof.

Over ten days beginning on August 31, 1854, cholera killed more than 500 people within 250 yards of the corner of Broad Street and Cambridge Street in Soho. Snow looked into the 83 deaths registered in the first three days and found that nearly all had occurred near the Broad Street pump or among people who drank its water. On the evening of Thursday, September 7, he put his findings to the Board of Guardians of St. James's parish, and the pump handle was removed the next day. By then the outbreak had passed its peak and deaths were already falling. Snow's map, drawn later for his book, marked each death with a black bar at its house and showed every public pump nearby; his table counted 616 deaths.

Snow was careful about what he could and could not claim. He did not know what caused cholera: Filippo Pacini described the bacterium in Florence that same year, but his work was ignored, and Robert Koch's expedition of 1883 and 1884 established it. What Snow had was an association specific enough that contaminated water was the only coherent explanation. He also used a natural experiment. The Southwark and Vauxhall company drew its water from the Thames downstream of sewage outflows, while the Lambeth company had moved its intake upstream to Thames Ditton. In the first seven weeks of the 1854 epidemic, houses supplied by Southwark and Vauxhall had 315 cholera deaths per 10,000, against 37 for Lambeth.

Henry Whitehead, a local curate initially skeptical of Snow's conclusion, later worked with him on the parish inquiry. Whitehead traced the probable first case to an infant at 40 Broad Street whose diarrheal discharges, diluted with water, were poured into a cesspool from August 28 to 30; the cesspool was found to connect with the well. Snow published his full analysis, with the pump map and the Southwark and Lambeth comparison, in the second edition of "On the Mode of Communication of Cholera," dated 1855.

Snow died in 1858, and his conclusion was largely ignored in his lifetime. Whitehead argued in 1867 that removing the handle had not stopped the outbreak, which had already run its course, but had probably prevented a second one. The infant's father fell ill with cholera on September 8, the day the handle came off, and his discharges went into the same cesspool.