Cholera and the birth of epidemiology

Summary

• Cholera is a gut infection caught from water or food contaminated with the bacterium Vibrio cholerae. It causes severe diarrhoea and can be fatal within hours, but most patients survive if they are given oral rehydration solution, a drink that replaces the water and salts they lose.
• In the 1850s most doctors blamed disease on miasma, foul air rising from rotting waste. Snow believed cholera spread through drinking water fouled by sewage. With no germ yet identified, he had to prove it by counting who fell ill and what they drank, the method now called epidemiology.
• Cholera broke out around Broad Street in Soho on the night of 31 August 1854. Snow marked each death on a street map and found them clustered around a public pump.
  • He took his evidence to the parish Board of Guardians, which ran local services, on 7 September, and the handle was removed the next day.
  • His final count was 616 deaths in the outbreak.
• In south London two water companies ran pipes down the same streets. The Southwark and Vauxhall Company drew from the Thames where sewage flowed in, and the Lambeth Company had moved upstream in 1852. In 1854 the first company's customers had 315 cholera deaths for every 10,000 houses, and the second company's had 37.
• The inquiry into the 1854 epidemic was not persuaded. It wrote "After careful enquiry we see no reason to adopt this belief" and favoured air as the route. Official opinion changed after London's last cholera epidemic in 1866, eight years after Snow died.
• Outside Britain cholera still spreads. In 2025, 47 countries reported 7,870 deaths and 451,499 cases to the World Health Organization, the most deaths since 1999. WHO says the true toll is higher.

Dig deeper

• Snow's own book admits the outbreak was already fading. Attacks fell from 143 on 1 September to 12 on the day the handle came off, partly because many residents had fled. He wrote that it was "impossible to decide" whether the well still carried the disease by then.
• Henry Whitehead, a local priest who doubted Snow, traced the likely first case to a baby at 40 Broad Street whose nappies were rinsed into a cesspool, a pit for household waste, by the house. In 1855 the parish surveyor found the cesspool's brickwork was defective, so sewage could seep towards the well.
• London's new sewers were built to get rid of a smell. Officials still believed sewage caused the bad air that spread disease, and in the hot summer of 1858 the stench from the Thames, known as the Great Stink, led the engineer Joseph Bazalgette to build a new system.
  • The system also kept sewage out of the drinking water. The 1866 epidemic struck parts of east London not yet connected to it, and the statistician William Farr traced it to a water company's contaminated reservoirs.
• Campaigns to vaccinate people before an outbreak stopped in 2022 and restarted in February 2026, beginning in Mozambique, because a global rise in cases had used up the supply. Outbreak teams still give one dose, which protects for at least six months. Two doses protect for about three years.
• In 2025 more than one in five deaths happened outside a health facility. Seven countries, among them Sudan and Yemen, had about 90% of cases and deaths. WHO names conflict, mass displacement and natural disasters among the conditions that help it spread, the same conditions described in Healthcare in war zones.
• In the UK, cholera is a notifiable disease, one that doctors must report by law. A doctor who suspects it must phone the UK Health Security Agency within 24 hours without waiting for a laboratory result. The agency, which tracks infectious disease in England, then looks for where the infection came from.

Ethical issues

Interview questions

• What do you know about John Snow and the Broad Street pump?
• What is epidemiology, and how did the study of cholera help to create it?
• Cases in Soho were already falling when the pump handle was removed. Does that weaken Snow's case?
• Snow linked cholera to dirty water before anyone had identified the germ. Should doctors act on a pattern in the numbers before they know the cause?
• You suspect a patient back from abroad has cholera, and they ask you not to tell anyone. What do you do?
• With too little vaccine for an outbreak, what are the arguments for giving one dose to twice as many people, and for giving two doses to half as many?
• Why do people still die of cholera when its cause has been known for over 150 years?
• London's sewers were built to clear bad air, and the city had no cholera epidemic after 1866. What does that tell you about evidence and public health?