Although the concept of a healthy carrier was not well understood in 1909, some physicians were aware of this special type of condition. They knew it was possible that a person could spread the infection without any outward signs of the disease. "While this seemed a likely theory, no one had ever identified an actual chronic carrier," according to an article in American Heritage (Gordon 120). People who were thought to be infected were often treated as pariahs and sometimes quarantined for long periods until physicians could figure out what do with them. Soper suspected right away that Mary Mallon was a carrier. He began to investigate all of her previous employers.
Using her employment agency as a guide, Soper discovered that a trail of typhoid followed Mary Mallon wherever she went. In the previous ten years, eight households in which she worked as a cook developed typhoid either during her employment or shortly afterwards. In the most serious case, at Dark Harbor, Maine, seven of nine people became infected shortly after Mary was hired as a cook. When the children became stricken with typhoid, she stayed on and nursed them back to health. Her employer, a New York City attorney named J. Coleman Drayton, gave her a financial bonus to demonstrate his thanks. A later investigation into the source of the outbreak was inconclusive. And because Mary was not sick herself, she was never suspected as being infected with the dangerous bacteria.
In all, "Soper identified twenty-two cases of typhoid in his search of Mary Mallon's employment between 1900 and 1907...this number is actually quite small and indicates that many of the people for whom Mallon cooked during these years may have already been immune to typhoid by virtue of having recovered from the disease" (Leavitt 18). Soon, Soper began to track Mary herself in the hope he could find her and examine her for typhoid. "I supposed that she would be glad to know the truth and to be shown how to take such precautions as would protect those about her against infection," Soper wrote later. "I thought I could count upon her cooperation in clearing up some of the mystery which surrounded her past."
Using advertisements in newspapers, Soper contacted employment agencies and interviewed other cooks who may have known Mary. He interviewed hundreds of people in New York and more in Boston where many servants placed their ads for employment. On several occasions, he thought he had found her, only to discover that she had left employment a day or two before he arrived. But Dr. Soper was a persistent man. In March of 1907, he located the elusive Mary working as a cook on Park Avenue near E. 60th Street in New York City. "When informed that the woman he had been pursuing over the last months was indeed working only a few feet from his opulent home, Dr. Soper could hardly contain himself," writes Bourdain (33).
The elated doctor rushed over to Park Avenue, where he expected to find a gratified Mary Mallon who would thank him for stopping the spread of a deadly disease. What he found, however, was something entirely different.