Mary Mallon was born in 1869 in Cookstown, County Tyrone, in what is now Northern Ireland. Presumably, she was born with typhoid because her mother was infected during pregnancy. At the age of 15, she migrated to the United States. She lived with her aunt and uncle for a time and worked as a maid, but eventually became a cook for affluent families.

From 1900 to 1907, Mallon worked as a cook in the New York City area for eight families, seven of which contracted typhoid. In 1900, she worked in Mamaroneck, New York, where within two weeks of her employment, residents developed typhoid fever. In 1901, she moved to Manhattan, where members of the family for whom she worked developed fevers and diarrhea, and the laundress died. Mallon then went to work for a lawyer and left after seven of the eight people in that household became ill.

In June 1904, she was hired by a prosperous lawyer, Henry Gilsey. Within a week, the laundress was infected with typhoid, and soon four of the seven servants were ill. No members of Gilsey’s family were infected, because they resided separately, and the servants lived in their own house. The investigator Dr. R. L. Wilson concluded that the laundress had caused the outbreak, but he failed to prove it. Immediately after the outbreak began, Mallon left and moved to Tuxedo Park, where she was hired by George Kessler. Two weeks later, the laundress in his household was infected and taken to St. Joseph’s Regional Medical Center, where her case of typhoid was the first in a long time.

In August 1906, Mallon took a position in Oyster Bay on Long Island with the family of a wealthy New York banker, Charles Henry Warren. Mallon went along with the Warrens when they rented a house in Oyster Bay for the summer of 1906. From August 27 to September 3, six of the 11 people in the family came down with typhoid fever. The disease at that time was “unusual” in Oyster Bay, according to three medical doctors who practiced there. The landlord, understanding that it would be impossible to rent a house with the reputation of typhoid, hired several independent experts to find the source of infection. They took water samples from pipes, faucets, toilets, and the cesspool, all of which were negative for typhoid.

In late 1906, Mallon was hired by Walter Bowen, whose family lived on Park Avenue. Their maid got sick on January 23, 1907, and soon Charles Warren’s only daughter got typhoid and died. This case helped to identify Mallon as the source of the infections. George Soper, an investigator hired by Warren after the outbreak in Oyster Bay, had been trying to determine the cause of typhoid outbreaks in well-to-do families, when it was known that the disease typically struck in unsanitary environments. He discovered that a female Irish cook, who fitted the physical description he had been given, was involved in all of the outbreaks. He was unable to locate her because she generally left after an outbreak began, without giving a forwarding address. Soper then learned of an active outbreak in a penthouse on Park Avenue and discovered Mallon was the cook. Two of the household’s servants were hospitalized, and the daughter of the family died of typhoid.

Soper published his findings on June 15, 1907, in the Journal of the American Medical Association. He wrote:

On March 19, 1907, Mallon was sentenced to quarantine on North Brother Island. While quarantined, she gave stool and urine samples three times per week. Authorities suggested removing her gallbladder, but she refused because she did not believe she carried the disease. At the time, gallbladder removal was dangerous, and people had died from the procedure. Mallon was also unwilling to stop working as a cook, a job that earned her more money than any other. Having no home of her own, she was always on the verge of poverty.

Not all medical experts supported the decision to forcibly quarantine Mallon. For example, Milton J. Rosenau and Charles V. Chapin both argued that she just had to be taught to carefully treat her condition and ensure that she would not transmit the typhoid to others. Both considered isolation to be an unnecessary, overly strict punishment. Mallon suffered from a nervous breakdown after her arrest and forcible transportation to the hospital. In 1909, she tried to sue the New York Health Department, but her complaint was denied and the case closed by the New York Supreme Court. In a letter to her lawyer, she complained that she was treated like a “guinea pig”. She was obliged to give samples for analysis three times a week, but for six months was not allowed to visit an eye doctor, even though her eyelid was paralyzed and she had to bandage it at night. Her medical treatment was hectic: she was given urotropin in three-month courses for a year, threatening to destroy her kidneys. That was changed to brewers yeast and hexamethylenamin in increasing doses. She was first told that she had typhoid in her intestinal tract, then in her bowel muscles, then in her gallbladder.

Mallon herself never believed that she was a carrier. With the help of a friend, she sent several samples to an independent New York laboratory. All came back negative for typhoid. On North Brother Island, almost a quarter of her analyses from March 1907 through June 1909 were also negative. After 2 years and 11 months of Mallon’s quarantine, Eugene H. Porter, the New York State Commissioner of Health, decided that disease carriers should no longer be kept in isolation and that Mallon could be freed if she agreed to stop working as a cook and take reasonable steps to avoid transmitting typhoid to others. On February 19, 1910, Mallon said she was “prepared to change her occupation (that of a cook), and would give assurance by affidavit that she would upon her release take such hygienic precautions as would protect those with whom she came in contact, from infection.” She was released from quarantine and returned to the mainland.

In 1915, Mallon started working at Sloane Hospital for Women in New York City. Soon 25 people were infected, and two died. The head obstetrician, Dr. Edward B. Cragin, called Soper and asked him to help in the investigation. Soper identified Mallon from the servants’ verbal descriptions and also by her handwriting.

Mallon again fled, but the police were able to find and arrest her when she took food to a friend on Long Island. Mallon was returned to quarantine on North Brother Island on March 27, 1915.

Little is known about her life during the second quarantine. She remained on North Brother for more than 23 years, and the authorities gave her a private one-story cottage. As of 1918, she was allowed to take day trips to the mainland. In 1925, Dr. Alexandra Plavska came to the island for an internship. She organized a laboratory on the second floor of the chapel and offered Mallon a job as a technician. Mallon washed bottles, did recordings, and prepared glasses for pathologists.

Mallon spent the rest of her life in quarantine at Riverside Hospital on North Brother Island. Six years before her death, she had a stroke. She never completely recovered, and half of her body remained paralyzed. On November 11, 1938, she died of pneumonia at age 69. Mallon’s body was cremated, and her ashes were buried at Saint Raymond’s Cemetery in the Bronx. Nine people attended the funeral.

In a 2013 article in the Annals of Gastroenterology, the authors conclude: