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The Pump on Broad Street

True story · 3 min read

On the last day of August 1854, people in one corner of Soho, London, began to die.

It was fast and ugly. Cholera drains the body of water in hours; a healthy man at breakfast could be gone by the next morning. By Snow's count, more than five hundred people living within a few streets of each other died within about ten days, and most of the neighborhood had fled.

John Snow was a physician who lived nearby. Almost everyone he knew believed cholera traveled in bad air, a poisonous mist rising from garbage and rot. Snow had a hypothesis of his own. Years earlier he had noticed that the illness struck the gut first, not the lungs, and a gut disease, he reasoned, had to be swallowed.

Believing something is easy. Snow wanted to show it.

He walked the streets with a notebook and asked, house by house, who had died and where they got their water. Then he did something unusual for the time: he put it on a map. Each death became a small black bar drawn at the address. The bars piled up around a single spot, the water pump on Broad Street.

A map is not proof, and Snow knew the objections. So he chased the exceptions, because exceptions are where an idea either breaks or earns its keep. A workhouse nearby, with hundreds of inmates, had lost very few people. It had its own well. A brewery on the same street had almost no deaths among its workers. They had their own well too, and they were given malt liquor to drink. And in Hampstead, far from Soho, a widow died of cholera. Her family said she had so liked the taste of the Broad Street water that she had it carried to her door every day.

On 7 September Snow took his evidence to the local Board of Guardians. They did not accept his theory of how the disease worked. But they agreed to act, and the next day the handle was taken off the pump. Nobody could draw water from it.

Here the story usually ends, with the epidemic stopping as if a tap had been turned. The truth is less tidy and more interesting. The outbreak was already fading by then, partly because so many people had left. Snow himself said as much. One removed handle cannot, on its own, prove a theory.

Stronger evidence came from the person who knew the street best. Henry Whitehead was a young curate who had spent years visiting these families, and at first he had thought Snow was wrong. He went back door to door, listening harder. He traced the earliest case he could find to a five-month-old girl at 40 Broad Street, a few steps from the pump, who had fallen ill days before the outbreak exploded. Her family had washed her soiled diapers and emptied the water into a cesspool. When the cesspool was examined later, it was found to be leaking, only a few feet from the well. The link was an inference, but a strong one. It showed how a single house could have contaminated the water for a whole street.

No one in London had seen the cause itself. An Italian scientist, Filippo Pacini, described the cholera bacterium that same year and was ignored; it was Robert Koch's work in 1883 that settled it. Snow died in 1858 without seeing his idea accepted. But his method outlived him: ask exactly who got sick, find what they share, and test the cases that do not fit.

That is still how epidemiologists work. Whenever someone traces an outbreak back to a single meal or a single water supply, they are using it.

Words in this story: epidemic, hypothesis, contaminated, evidence, cesspool