John Snow's Ghost Map

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Katie Lambert: Hello, and welcome to the podcast. I'm Katie Lambert joined by Sarah Dowdey. How are you, Sarah?

Sarah Dowdey: I'm great, Katie. How are you?

Katie Lambert: You won't be as great once we start talking about what we're talking about, which was a listener request. Jamie from DC wanted to hear all about John Snow and the cholera outbreak in Victorian London.

Sarah Dowdey: I don't know. I'm kind of a fan of slummy Victorian London, so I think I'm gonna enjoy this one. John Snow was born in 1813 in Yorkshire, England, and he was actually the son of a coal yard laborer, but quickly gets into the medical field at 14, when he starts three consecutive apprenticeships and first encounters cholera not that long after while visiting coalminers.

Katie Lambert: In 1831, he got his first exposure to contagious disease. He doesn't begin his formal medical education until 1836, but he gets his MD in 1844 from the University of London. By 1849, he is a licensed specialist at the Royal College of Physicians of London, which was a really elite organization.

Sarah Dowdey: Yeah, this guy gets big fast. He kind of enters the realm of what today we'd probably call a celebrity doctor, especially when he treats Queen Victoria, and that's because he learned about ether being used in America, but pioneered how it was dispensed.

Katie Lambert: Right. So he helps Queen Victoria through her childbirth on the birth of Prince Leopold and Princess Beatrice and makes the public more accepting of the process at all.

Sarah Dowdey: Yeah, if Victoria is into it, whether it's Christmas trees or big white weddings or ether, the kingdom likes it too.

Katie Lambert: Trendsetter Victoria. But we don't think of him today for his work in anesthesia. We think about him for his pioneering work in germ theory. To do that, we'll give a little background first about Victorian London, which was really disgustingly dirty.

Sarah Dowdey: It was a really nasty place, yeah.

Katie Lambert: The life expectancy for a gentleman in Victorian London was 45, but if you were a tradesman, it was your mid-20s, so I would be killed off by now and so would you. And London was really, really stinky, like known worldwide for being stinky. It was the biggest city in the world, but sewage was just piled up everywhere. Toilets drained into basement cesspits, so there would just be piles and piles of sewage in your basement stinking to high heaven.

Sarah Dowdey: And the cesspits were flushed into the river, if they were cleaned.

Katie Lambert: Which, of course, is where everyone got their water? Maybe you can see where this is going.

Sarah Dowdey: Yeah, so needless to say, London, overcrowded, dirty, stinky, a good place for diseases to spread.

Katie Lambert: And at the time, it was thought of that diseases came from miasma, or bad air from decayed organic matter, so bad smells meant disease. And when the entire city stinks, people thought you were getting sick from the sewage.

Sarah Dowdey: There was also a moral element to the whole miasma theory that stinky people were unclean and more prone to disease, not just because they were poor and destitute and living in overcrowded hovels. They were morally unsound.

Katie Lambert: So too bad for you, poor people who lived in Victorian London. Everyone thought your illness was your own darn fault. So Snow doesn't buy this, though. He thinks that diseases are caused by some agent, not by a smell.

Sarah Dowdey: Right, so they started calling that germ theory, and so we've got th ese two philosophies that are kind of going head to head, and they go head to head for decades, surprisingly enough to us, who miasma sounds like such a bad idea, but people aren't quick to buy germ theory.

Katie Lambert: There had been a few outbreaks of cholera in London. The 1848, 1849 cholera outbreak killed 50,000 people, and this is where John Snow wants to figure out how this is happening. Cholera is not a pretty disease. You die from basically diarrhea that's unstoppable and various digestive ills. You die from dehydration because your body doesn't have any fluid left.

Sarah Dowdey: And you can die really quickly, like within a day.

Katie Lambert: So John Snow wants to figure out this germ theory and see if he can prove it, but in that particular outbreak, there weren't any public death records, and he couldn't figure out who was giving water to which households, so this wasn't a good test case for him. And in the summer of 1854, another cholera outbreak happened. 700 people are dead in two weeks, and this is when he starts his experimentation and runs around testing water and interviewing people and trying to figure out where this is coming from so he can stop it.

Sarah Dowdey: And he performed two classic experiments during this 1854 outbreak. The first was the Broad Street pump outbreak experiment, which is my favorite. He's like the Sherlock Holmes of medicine in this thing.

Katie Lambert: He is. He's fascinating.

Sarah Dowdey: It's pretty amazing. So in the Soho district of London, where he's actually based - his medical offices are actually based - there is a sudden case of cholera, 70 fatalities within a 24-hour period, most of them within five square blocks. All of these fatalities are based around the Broad Street pump, which is a free water pump for the poor. It draws the water from a well underneath the Golden Square, which has some of London's poorest, most overcrowded people.

Katie Lambert: So in the last week of August 1854, all the residents of Golden Square start dying, and it starts with an upset stomach, and then goes to vomiting and severe cramps in the gut, and then to diarrhea and thirst, and then, like we said, death from dehydration. It's fast to kill. Some people are dying within 12 hours after it starts, and it's really fast to spread.

Sarah Dowdey: So the medical authorities are pretty quick to identify this as cholera, and Snow moves in to start studying what's happening. He takes a really multi-discipline approach. He looks at water samples and sees what he can find in the water, but he also starts looking at the maps of London dead and the weekly statistics about who's dying of cholera in London, looking for geographical patterns. He draws a ghost map that showed a correlation between cholera cases in this neighborhood and the Broad Street pump. Basically, if you lived within walking distance of the Broad Street pump, if that was your nearest water source, you were very likely to come down with cholera.

Katie Lambert: And it's really intense. You can find a bunch of them online - of the ghost maps. But there are just black lines everywhere showing people dying.

Sarah Dowdey: They're very disturbing, little stacks of black lines, and you'll see the pump location and the houses immediately adjacent to the pump just have these huge stacks of black lines coming from them.

Katie Lambert: This is part of the reason he's called the father of modern epidemiology. It starts right here.

Sarah Dowdey: So after about a week, he goes to the local board of guardians of St. James Parish with his findings with this ghost map and convinces them to shut down the Broad Street pump, to literally take the handle off the pump.

Katie Lambert: So people can't use it.

Sarah Dowdey: They're not totally into it, though, are they?

Katie Lambert: No, they're still thinking about the whole miasma thing, so they're engaged in this pursuit to spread lime all over the streets because that'll kill the smells and surely, that will kill it.

Sarah Dowdey: Get the bad air out.

Katie Lambert: But they decide, okay, the guy is convinced.

Sarah Dowdey: Go along with the experiment.

Katie Lambert: So let's go ahead and take the pump handle off. And surprise, surprise, the outbreak ends.

Sarah Dowdey: But what's so great about Snow's experiment here is he doesn't just look at the overwhelming evidence on the side of if you drink this water, you very well might get sick. He looks at kind of the statistical outliers.

Katie Lambert: Yes, he's very thorough. I love this.

Sarah Dowdey: Yeah, he - there's some school children who don't live near the pump who end up dying. He reasons that they passed by the pump on their way to school.

Katie Lambert: And my favorite, there's a widow in west end Hampstead and her niece in Islington, and they got sick, but neither of them had been anywhere near Soho. So he did some investigation, did some interviewing and discovered that the widow had once lived on Broad Street and liked the taste of the well water so much that she had a servant go to Soho every day and bring her back a bottle of it to drink.

Sarah Dowdey: It's like when you go to Florida or South Georgia, and you bring your Atlanta water.

Katie Lambert: Bring your own water.

Sarah Dowdey: So yeah, he actually finds the last bottle of water that the widow had gotten was from August 31, which is the start of the epidemic, so bad timing there. There's also an army officer living in St. John's Woods who dies after dining in Wardour Street where he had drunk a glass of water from the Broad Street well.

Katie Lambert: And he also, in his thoroughness, looks at the people who didn't get sick, so the people at the Poland Street workhouse are just around the corner from the Broad Street pump. If you're thinking about it, they should have been sick, but they weren't. So he went and looked into that, and that's because the workhouse had its very own water source. They weren't using the Broad Street pump.

Sarah Dowdey: And that's also a good case against the miasma theory.

Katie Lambert: Right.

Sarah Dowdey: These people are in the workhouse. They're dirty. They're more likely to be morally suspect.

Katie Lambert: Morally corrupt. Sorry, y'all.

Sarah Dowdey: But here they are safe from cholera. Also, the Broad Street Brewery, which is right down the street from the pump, no deaths because the workers are given a daily beer allowance, so they don't need to drink water. Feel like there's a lesson in there somewhere for my bosses?

Katie Lambert: He also has the help of Reverend Henry Whitehead, who's the vicar of St. Luke's Church. Whitehead actually wasn't originally on his side. He thought the outbreak was caused by God's intervention, and he started a report to prove it, but it actually only ended up confirming John Snow's study.

Sarah Dowdey: But he was man enough to come to Snow and admit, "My research is the same as yours."

Katie Lambert: To admit it.

Sarah Dowdey: And he actually helped Snow track down the source of the local outbreak. A sick child at No. 40 Broad Street right near the pump had had his diapers washed, and the water was dumped into a cesspool that was only a few feet away from the well.

Katie Lambert: And after the child died, no more diaper pail water had been dropped in tha t cesspit, so people stopped getting sick.

Sarah Dowdey: Yeah.

Katie Lambert: So later in the year, our Sherlock Holmes, John Snow, conducts a grand experiment, and he compares the London neighborhoods who are receiving water from two different companies, and one company uses water that comes from the upper Thames, and the other uses water that comes from the heart of London.

Sarah Dowdey: Interestingly, parliament had actually required the metropolitan water companies to improve the quality of their intake, but not all of them had complied.

Katie Lambert: And, of course, sewage is being dumped into the Thames. The sanitation commissioner named Edmond Chadwick believed in the miasma thing, and he thought that if you dumped sewage in the river, you were keeping bad air away from people. He thought what he was doing was actually really good, but of course, he's dumping sewage into water that's then getting turned into drinking water.

Sarah Dowdey: This dual water company thing kind of presents the perfect opportunity for an experiment for Snow because the companies were rivals and had at one point competed head to head. So some houses had mains from one company, while their next door neighbor had a main from the other company. So essentially, you had this controlled experiment.

Katie Lambert: Right.

Sarah Dowdey: Everything was the same in this neighborhood except for the water - where they got their water from. It turned out for people who got the London sourced water; they had a much higher chance of contracting cholera.

Katie Lambert: And Snow is overjoyed because he thinks, wow, he's finally proved it. The ratio of people who died from one source of water versus the other was something ridiculous, like 75 to 5. If that's not proof, what is? He suggested intervention strategies to control epidemics, and he thinks that he's proven that contaminated water is what gets people, but it didn't seem to stick.

Sarah Dowdey: No, people are still stuck on the miasma theory. Sadly, it's not really until the 1880s when germ theory is golden, and people go with that when the causative organism of cholera, Vibrio cholerae, is actually finally understood.

Katie Lambert: So when John Snow died in 1858, people still thought it was miasma and no one accepted all the things he'd worked on so hard. Chadwick was still suggesting ridiculous things. At one point, he was quoted as saying, "All smell is, if it be intense, immediate, acute disease." And in the 1890s, he suggested bringing down fresh air from places like the Eiffel Tower and distributing it to [inaudible].

Sarah Dowdey: Katie and I were discussing how that would actually be done. How do you catch the air and then distribute it like a ration?

Katie Lambert: Unfortunately, I can't ask Mr. Chadwick. The Great Stink of 1858, which is my favorite name of anything that has ever happened ever, is what starts to change things because this summer was incredibly hot, and sewage was everywhere in London. The flush toilets were overflowing. The basement cesspits, which were going into the street drains, and it was so bad no one wanted to be in the city. It was so horrible the people in the House of Commons were draping their curtains and soaking them in chloride of lime just so they wouldn't be smelling the sewage. So a committee was set up to figure out how to fix the stink, and this is where the modernization of the sewage system in London started to happen.

Sarah Dowdey: So even though sanitation is much better in London today, it's still a problem in a lot of places in the world, and cholera is actually still causing a lot of deaths.

Katie Lambert: Diarrhea is one of the leading causes of death for kids in the developing world.

Sarah Dowdey: There's a treatment for it today, oral rehydration salts, which basically keep you from dying of dehydration in 12 hours, 24 hours. It's estimated that it's prevented 40 million deaths since 1978.

Katie Lambert: So today, we'd like to give thanks to John Snow and his amazing investigative work, and we recommend the book The Ghost Map if you haven't picked it up. And if you'd like to learn more about infectious diseases and safe water supply, come to How Stuff Works and also check out the blog on our home page at

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