Mary Mallon, commonly known as Typhoid Mary, was an Irish-born American cook who is believed to have infected between 51 and 122 people with typhoid fever. The infections caused three confirmed deaths, with unconfirmed estimates of as many as 50. She was the first person in the United States identified as an asymptomatic carrier of the pathogenic bacteria Salmonella typhi. She was forcibly quarantined twice by authorities, the second time for the remainder of her life because she persisted in working as a cook and thereby exposed others to the disease. Mallon died after a total of nearly 30 years quarantined. Her popular nickname has since become a term for persons who spread disease or other misfortune, not always aware that they are doing so.

A quarantine is a restriction on the movement of people, animals and goods which is intended to prevent the spread of disease or pests. It is often used in connection to disease and illness, preventing the movement of those who may have been exposed to a communicable disease, yet do not have a confirmed medical diagnosis. It is distinct from medical isolation, in which those confirmed to be infected with a communicable disease are isolated from the healthy population. Quarantine considerations are often one aspect of border control.