A nurse recieves a typhoid vaccination in 1915. Even if she's eating something Typhoid Mary serves up, she should be safe.

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Typhoid fever isn't­ a p­retty disease. Painful diarrhea, high fever, nasty red rashes and sleeplessness typically characterize the illness. Left untreated, typhoid can result in death. Salmonella typhi, the parasite that causes typhoid fever, spreads through water and food, making the disease highly contagious. Those who don't know the whole story are quick to blame one individual, known to history as Typhoid Mary, for intentionally spreading the deadly illness. As we'll see, the truth is a little more complicated.

In turn-of-the-century New York City, typhoid was a growing problem. The Department of Health had a lot on its plate; in addition to typhoid, it was trying to quell out­breaks of smallpox, tuberculosis, diphtheria and whooping cough that were sweeping through the area [source: NOVA]. Luckily, scientists had developed a sophisticated understanding of microbial diseases and how they spread -- even if everyone in the lay public didn't quite grasp all of it yet.

The Department of Health knew what caused typhoid, but dealing with the spread of the disease was another question altogether. It's a question that plagues us to this day. It's no longer considered humane to simply cast contagious disease victims out of society and into the wilderness to fend for themselves. What exactly to do with them remains controversial. Authorities must walk the line between keeping their societies safe from debilitating illness and infringing on the victims' personal rights. This controversy reached a fever pitch in early 20th-century New York when it came to one individual.

It might surprise you to learn that this fervor revolved around someone who was actually immune to typhoid. Though it's uncommon, some people are naturally immune to the illness, meaning they can carry the parasite and never suffer from a single symptom. Nevertheless, these people can just as easily spread the disease to others. This was the case for one Mary Mallon, aka Typhoid Mary. She was in the wrong place at the wrong time as well as in the worst possible occupation for a carrier of typhoid: She was a cook.