Mary Mallon was born in Ireland in 1869. When she was a young teenager, she left for New York City, where she lived with her aunt and uncle until their death. Despite being alone in a large metropolis in a new country, Mary learned to make a good living as a servant in various households. By her 30s, she earned a position as cook, in which she served a dessert of ice cream and peaches that was to die for (quite literally, as we'll see).
When typhoid fever broke out in the upscale Oyster Bay community, one family brought in Dr. George A. Soper, an epidemiologist and sanitation engineer, to investigate what was going on. He was well-versed in the disease and aware of cases involving immune carriers. He asked the family for details about their eating habits as well as the staff who prepared their food. He investigated the possibilities that the illness transferred through oysters or that the sewage pipes could have tainted the drinking water.
When these proved dead ends, he focused on the kitchen staff. He finally pinpointed Mary as the likely culprit. After investigating her work history, he found that the families she worked for in the past had suffered from typhoid outbreaks as well. Had she used the bathroom without washing her hands and then prepared food for the family, she would've been able to spread the disease. Soper learned that most of the food she served was cooked (and thus probably safe from typhoid) but that her trademark ice cream and peaches dessert could have very likely infected the family. By this time, however, she was no longer working with the family that had hired Soper. And because she never left forwarding addresses when she left a household, it took some sleuthing to track her down.
When Soper did find her, the tough Irish cook was unwilling to cooperate. Soper tried explaining to Mary that she was infecting families with her cooking and that he needed to check her feces and urine for the disease. He may have explained the situation indelicately, however, because the story goes that before he could finish, Mary chased Soper from her kitchen with a carving fork.
Nevertheless, Soper was determined to test her for the illness, even if he had to drag her away kicking and screaming -- which was exactly what happened.