At the turn of the 20th century, scientists knew about contagion, but the concept of an asymptomatic carrier was completely new. How could someone spread a disease when they weren't sick? We can't say that Mary Mallon, also known as Typhoid Mary, was the first asymptomatic carrier, but she was the first that health officials knew about. Determining how all these wealthy families families came to contract typhoid would be easy now, since they all hired the same cook, but inspectors at the time were looking for actual sick people or bacteria in the water supply. George Thompson owned a vacation house in which the Warren family became sick, and he was determined to find the source to protect future renters.
Desperate to get to the bottom of the puzzle, Thompson hired George Soper, thirty-seven-year-old freelance civil engineer who had been investigating typhoid outbreaks in well-to-do families. After learning that the Warrens had hired a new cook, who no longer worked with them, Soper had his suspicions. Soper was able to trace Mary's employment history back to 1900. He found that typhoid outbreaks had followed Mary from job to job. From 1900 to 1907, Soper found that Mary had worked at seven jobs in which 22 people had become ill, including one young girl who died with typhoid fever shortly after Mary had come to work for them.
Soper tracked Mary down to her new place of employment, the family of Walter Bowens, who lived on Park Avenue. There was typhoid in the residence too. Soper found that two of the household's servants were hospitalized, and the daughter of the family had died of typhoid.
Soper confronted Mary in the kitchen of the Bowens, and asked her to give samples of her urine and stool. This infuriated Mary. Grabbing a carving fork from the table, Mary chased Soper out of the house. Sopher tried again, this time in the apartment of a man Mary was spending time with. Mary threw Sopher out again, swearing the whole way.
If she had cooperated, the name Mary Mallon might only be known in science and medical circles, but she fought quarantine for years and became so notorious that her nickname is still used for people who spread disease. Read the story of Typhoid Mary at Amusing Planet.