She spent the summer working for the Warren family while they vacationed in Oyster Bay, Long Island. Shortly after she left, three family members, two maids, and a gardener all came down with typhoid fever. The owner of the property hired George Soper, a sanitation engineer, to find the root cause. Soper hypothesized that it was Mallon, after learning that all of the families she had worked for had had outbreaks at some point. He asked to test her stool and urine but she refused, as she felt perfectly healthy (asymptomatic carriers were unheard of at the time). Soper continued to plead with her, even offering to pay her royalties from a book he wanted to write about her. Mallon just wanted to be left alone and told him that she had been tested privately and the results were normal.
The rest of Mallon's life was spent in a struggle between her right to do as she pleased and the authorities who were sure she was spreading the deadly disease. Read what happened to Mallon at Atlas Obscura. Link