term used to describe someone who, knowingly or not, carries
The term originated from a woman named Mary Mallon. She lived and worked in New England and New York around the turn of the century. She acted as a cook and housekeeper for several wealthy families, members of which became very ill with typhoid. A doctor named George Soper finally pieced things together, and interviewed Mary about the families she'd worked for. She refused any connection between herself and their sickness.
Mary was arrested by New York public health officials, and stool and urine cultures were taken by force. She was found to be teeming with Salmonella typhi, the bacterium responsible for typhoid fever. Mary was labeled a public health threat and confined to a cottage in the Bronx. She worked as a laundress until 1910, when she was released with the promise that she never work as a cook again. She lied, changed her name, and obtained various jobs as a cook, even at New York's Sloan Hospital for Women. There, she infected at least 25 people, resulting in two confirmed deaths.
She was arrested again after the Sloan Hospital case, and went without a fight. She was quarantined for life on North Brother Island, where she died in 1938. An autopsy showed she was still infested with Salmonella typhi.