Mary Mallon was a thirty-one-year-old immigrant from northern Ireland who spent much of the first decade of the twentieth century cooking for wealthy families in New York. Mary had eight jobs in seven years, and typhoid fever— a disease that results in the inflammation of the small intestine, fever, coughing, abdominal pain, and diarrhea — followed in her path. Although never sick herself, she spread the disease in the households where she worked and beyond. At least fifty-three cases and three deaths can be attributed directly to Mary Mallon, but she may have been responsible for fourteen hundred cases in Ithaca, New York, in 1903. Epidemics of the disease at the time could cause thousands of cases and many deaths.

Mary Mallon was the first carrier of typhoid to be identified. Public health officials persuaded her to enter Riverside Hospital in the Bronx section...