on how nobody knew how to prevent cholera. You might suspend acorus over your masked

mouth, or wear a copper breastplate, and tie pitchers of water behind your calves.

Anything to keep the disease away. Starting in the 1830s, cholera pandemics swept

the United Kingdom. Nobody knew how the disease was transmitted.

Germs weren't an established idea. One London doctor — John Snow — tried

to find out how the diseases spread, and today, one of his investigations is iconic in the

field of epidemiology. And it all centered on a pump.

This is a map John Snow made to prove his solution to the cholera mystery in London.

It also shows the confusion and the problems he was up against.

Each of these bars represents a death from cholera.

The disease often killed half the people who got it — it caused vomiting and diarrhea.

The rapid loss of fluids was fatal. At the time, a lot of people believed cholera

was transmitted in a “miasma” — imagine an evil cholera cloud.

This typical map from the 1840s shows a cholera “mist” that was blamed for transmission.

Snow suspected a different source. At the time, people usually didn't get water

directly in their homes. It came from a neighborhood pump connected

to one of the few water companies in the city. John Snow mapped different water company's

service areas in London. You can see how they are occasionally separate,

and occasionally overlap. If a common pump was contaminated at any point

— at the source or near the pump — Snow believed the water could kill.

In 1849, he wrote that his study of symptoms and specific cases had led him to suspect

'the emptying of sewers into the drinking water of the community," caused outbreaks

— not a miasma. Five years later, he had a chance to prove

it — and stop a fresh outbreak in the process. In August 1854, 20 people lived here at 40

Broad Street, including an infant who died of cholera. After her death, Snow started

to investigate the outbreak. He didn't think the original water source

was the problem, but he thought something might be wrong down the line, at the pump.

He took samples of the water. They seemed clean.

But he wasn't satisfied, because more people were getting sick.

He asked questions up and down the street, where one man had noticed a bad smell from

his water. Snow asked the registrar for a list of people

who'd died. He started going house by house to interview

the survivors - and many of the dead had taken water from the pump.

He became convinced the Broad Street Pump was the common link among the dead.

He wrote, “I had an interview with the Board of Guardians of St. James's Parish on the

evening of Thursday, 7th September, and represented the above circumstances to them. In consequence

of what I said, the handle of the pump was removed on the following day.”

People stopped using the water. But Snow had not won yet. Newspapers reported

the streets were covered in lime — the city was using it as “a powerful disinfectant”

on the streets. That showed they weren't fully convinced

the pump was the problem. They still suspected miasma.

So Snow bolstered his case through investigation and recording.

He learned the 18 workers who died at this factory had drunk from big barrels of water

drawn from the pump. At the same time, unlikely survivors could

serve as proof of Snow's theory. According to the miasma theory, this place

would've been covered in cholera clouds, affecting all workers.

But Snow learned the workhouse had its own well — no bad water got in.

The same went for this brewery. That's because Snow learned the workers

there drank from the brewery's water supply or, more likely, only drank the free malt

liquor they got on the job. That's right, drinking on the job saved

their lives. Snow strengthened his argument and his map.

He adjusted the location of the pump to show how close it was to 40 Broad Street and drew

a dotted line - he showed a zone where it would be closest to walk to the Broad Street

pump, rather than another one. That zone is where most people died.

He tabulated every death, by date, to do it. This was paired with a local Reverend's

similar data-driven investigations. A local surveyor looked at the plumbing at

40 Broad Street, where the infant had died. He learned that the cesspool, where sewage

collected, was poorly designed and lined with decaying bricks.

When the infant's diapers had been washed, the cholera-carrying water had leaked into

the Broad Street pump's supply. John Snow died in 1858. His obituary read,

“Dr John Snow: This well-known physician died at noon, on the 16th instant, at his

house in Sackville Street, from an attack of apoplexy. His researches on chloroform

and other anaesthetics were appreciated by the profession.”

At the time, Snow was more famous for stuff like a chloroform inhaler than a map.

It took years for the “investigation of John Snow” to become an example for subsequent

outbreaks and epidemiology textbooks, and it slowly, eventually, helped end the miasma

myth. That's because Snow didn't just make a

map of a city. It's a map of his process and the field

it shaped. It gave direction to a world where disease

didn't have to be hidden in a cloud. Instead, it could start at a pump.

OK, so the

best book about John Snow is Cholera, Chloroform, and the Science of Medicine. It features an

amazing story, which is that John Snow gave chloroform to Queen Victoria while she was

giving birth.