This story is over 5 years old.

Doctors Didn’t Know How Cholera Spread Until One Genius Drew a Map

In the midst of a deadly cholera outbreak in London, a local anesthetist figured out how the disease was spreading using data visualization.
Image: Wikipedia

If you like mysteries, you'll love our podcast Science Solved It. The second season just started. Click here to listen for free.

Cholera can dehydrate your body so rapidly that you die within just a few hours of first feeling sick. The horrible disease still plagues people today—there's currently a widespread outbreak in Yemen—but we're a lot better at controlling it, thanks to a discovery that was made almost accidentally by a doctor who drew a map.


Dr. John Snow (not that Jon Snow) was an anesthetist that lived in London in the 1850s. He was a well-regarded doctor who even attended to the Queen when she gave birth, but his claim to fame more than a century later is his discovery during a deadly outbreak of cholera in 1854.

Listen to the full episode about John Snow here:

"Pandemic cholera started in the Sundarbans in the Bay of Bengal—in what is now Bangladesh—and that started around 1817," explained Sonia Shah, a science journalist and the author of Pandemic. "It slowly went up with traders and soldiers and travelers, came up into Russia and slowly filtered into the cities of the old world, which were, of course, rapidly industrializing."

Cholera arrived in London in the 1830s, during the height of industrialization when hoards of people were moving into the city. Many parts of London had poor sanitation, and no indoor plumbing, which meant human waste was disposed of in cesspools—basically holes dug underneath houses—which drained into the river. A lot of neighborhoods smelled really bad, which leant credence to the widely-held theory for how cholera spread: miasmas.

Miasmas was a theory that the air that rose up from decomposing waste was somehow infected, and if you breathed in that air, it would make you sick. This is how doctors at the time believed cholera was spreading. But this theory never quite made sense to Snow.

"He's an anesthetists, so he knows how gases move around," Shah told me. "When you breathe in a gas or a miasma, what [should] get sick is your respiratory system. You breathe it in in your lungs, you'll cough or you'll be hacking. But with cholera, it wasn't your respiratory system that was getting affected. It was obviously your digestive system."


In 1854, a new outbreak of cholera was spreading in Soho, a densely-populated neighborhood of London near where Snow lived. He noticed that a lot of the outbreak seemed to be concentrated around one particular street corner, so he got an idea. With the help of a local priest, Snow went door to door in Soho, interviewing inhabitants and tracking how many people had gotten sick and died at each residence over the last few weeks. Then, he plotted that data on a map:

Each black line on this map represents a death that occurred at that particular household. When you look at this map, a pattern emerges of the disease's spread. It's like a ripple effect, emanating from one street corner, at Broad Street and Cambridge Street. And what was on that street corner? A water pump.

Through some more investigating, Snow was able to determine that a backed up cesspool with a sick baby's diaper had contaminated this water source, which was where 60 percent of the people who had gotten sick got their water from.

It became obvious to him that water was the key to how cholera spread, but the powers that be wrote him off. At best, the considered the idea possible in some rare circumstances, at worst, they thought he was completely wrong. It wasn't until the 1890s, years after Snow had died, and the emergence of germ theory, that doctors finally embraced the idea that cholera spread through water. But think how many lives could have been saved if they understood what the data was obviously showing decades earlier.

It's an important reminder that emerging science isn't always easy to accept, and sometimes challenges our understanding of the world. But if we're willing to look at the evidence and continue questioning our beliefs, we might be able to avoid a lot of unnecessary suffering.

Like science mysteries? Subscribe to Science Solved It , Motherboard's new show about the greatest mysteries that were solved by science.