Cobra Effect · Body and medicine
The pump that spread cholera
How a map of deaths around one pump showed that cholera spreads through water.
7 cards, read aloud in 3:01, with a test and sources.
In 1854, most doctors believed cholera spread through foul air.
Cholera killed quickly, with violent sickness, and swept through crowded cities. The leading theory said it came from miasma: bad air rising from rotting matter and filth. A London doctor, John Snow, thought otherwise. He suspected cholera was spread by something in the water.
At the end of August 1854, cholera broke out around Broad Street in Soho.
Within days, hundreds of people in the neighbourhood had died. Many families drew their water from a public pump on Broad Street. Snow went from house to house, asking where the people who died had got their water. The Reverend Henry Whitehead, who at first believed in bad air, helped him.
Snow marked the deaths on a map, and they clustered around the pump.
The closer people lived to the Broad Street pump, the more of them had died. Deaths were fewest where people had other water to drink. The pattern pointed to the water, not the air. Foul air could not easily explain a pattern centred on one pump.
Workers at a nearby brewery were spared.
The brewery stood close to the pump, yet none of its workers caught cholera. They were given beer to drink, and did not drink the pump water. Brewing involves boiling, which kills the germs. A workhouse nearby, with its own well, also escaped the worst.
In September 1854, officials took the handle off the pump.
Snow presented his evidence to the local authorities, and they agreed to remove the handle. After that, no one could draw water from it. The outbreak ended soon afterwards. Snow himself admitted it had already been falling before the handle came off, so the removal may not have been what stopped it.
Whitehead then found how the pump water had been poisoned.
A baby living near the pump had fallen ill with cholera just before the outbreak. Water used to rinse the baby’s nappies had been emptied into a cesspit. The cesspit was faulty, and it lay close to the well beneath the pump. The source of the outbreak had been found.
So when an outbreak has an obvious cause, ask what the pattern really points to.
Snow also compared two water companies in South London, and found far more deaths among households drinking water taken from a dirtier stretch of the Thames. Even so, officials rejected his theory after the epidemic ended. It took decades for the idea that cholera spreads through water to be widely accepted. A careful map of where people died beat the most respected theory of the day.
Sources
- 1854 Broad Street cholera outbreak, Wikipedia. Snow’s map, the pump handle, the brewery, and the baby at the source.
- John Snow: cholera and the Broad Street pump, London Museum. The outbreak in Soho, and how Snow traced it to one pump.
- John Snow, cholera, the Broad Street pump: waterborne diseases then and now, PMC review. What Snow showed, what he got right, and why it still matters.
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