See this guy? He is afraid for his life.
This drawing is an 1832 joke - it’s a riff
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.
