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Michael: If you’re one of the almost five billion people with access to clean water and good hygiene, there are a lot of reasons to be thankful. For one thing, you don’t have to worry about typhoid fever. The disease, which is caused by a bacteria called Salmonella typhi, is spread by infected food, water, or contact with a sick person’s feces or urine, and symptoms include vomiting, diarrhea, and a nasty rash.

But some people don’t have any symptoms at all. They’re carriers, and can still spread the disease, even though they aren’t sick themselves. Mary Mallon, better known as Typhoid Mary, was one of those carriers.

In the early 1900s, typhoid fever was mainly a problem in poor urban communities - but suddenly, about 3,000 people, including rich families, got sick. And it was all thanks to Mary Mallon. Mallon immigrated from Ireland to New York City in 1883 and started working as a cook for wealthy families.

We don’t know when she originally got typhoid fever, but she was definitely a carrier, and the families she worked for always got sick after she started cooking for them. But by the time someone came to investigate, Mary had always moved on to the next house. Her luck ran out in 1906, while she was working for a family in Oyster Bay, on Long Island in New York. The family came down with typhoid fever — which was totally unexpected, because Oyster Bay was home to the rich and famous, and typhoid was considered a disease that mainly affected the poor.

So the house’s landlord hired George Soper, a freelance sanitary engineer, to find the source of the outbreak. All the water sources in the house turned up clean, and Soper eventually started looking into Mary’s employment history. Seven families she’d worked for — a total of 22 people — had reported cases of typhoid fever, including one girl who had died. It didn’t take Sherlock Holmes to put the pieces together: Mary was a typhoid carrier.

Even if they feel fine, carriers carry the typhoid-causing bacteria in their cells for years, which means they can get other people sick, too. The carrier doesn’t feel sick, but the bacteria are still in their feces and urine, so they can spread the disease if they don’t wash their hands well.

But even though Mallon was a cook, at first it didn’t make sense that she’d spread typhoid so easily. Even if she didn’t wash her hands, any bacteria in her food should have died from the high temperatures involved in cooking. Except it turned out that she was preparing non-cooked foods, too — one of her most popular dishes was peach ice cream, which was packed with raw peaches. After Soper’s investigation, Mallon was forced to have her feces tested for typhoid. The test was positive, so she was quarantined in a small shack on North Brother Island, a tiny island east of Manhattan.

Mary Mallon was the first known typhoid carrier in the United States, and her case wasn’t handled too well. Health officials didn’t do a great job explaining to Mallon why she was spreading disease even though she didn’t feel sick, and they didn’t prepare her for life as a carrier. Three years later, she was released from quarantine, but she had to promise not to go back to cooking. Mallon didn’t have a way to make a decent living doing anything else, though, so when the city lost touch with her, she went right back to the kitchen.

Five years later, typhoid fever broke out at Sloane Maternity Hospital in Manhattan, infecting 25 workers and killing two people. Health officials went to the hospital... and found Mary Mallon working there as a cook under a new name, Mary Brown. So she was taken back to North Brother Island, where she was quarantined for more than 23 years, until she died in 1938.

Even though Typhoid Mary was the first known typhoid carrier in the U.S., she wasn’t the last. Plenty of other New Yorkers carried the disease, but since Mary had gone back to cooking despite knowing she could infect people, the public turned against her.

These days, typhoid can usually be treated with antibiotics, and there are vaccinations for people who live in or travel to parts of the world where it’s common. And hopefully, as we keep working to get clean water and sanitation to people who need it, typhoid fever will become less of a problem. Carriers are kept under close watch, but as long as they aren’t working in jobs like cooking, they’re free to live their lives. Mary Mallon wasn’t so lucky.

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