An asymptomatic carrier (healthy carrier or just carrier) is a person or other organism that has contracted an infectious disease, but who displays no symptoms. Although unaffected by the disease themselves, carriers can transmit it to others.
In humans, HIV goes through a long latency period, during which the host is asymptomatic. Many carriers are infected with persistent viruses such as EBV and Cytomegalovirus that only rarely progress to a disease state. Herpes simplex viral infection may also be asymptomatic and can be spread without the originally infected person realising they are infected.
C. difficile has also been shown to be spread by asymptomatic carriers, and poses significant problems in care home settings.
Mary Mallon, known as "Typhoid Mary", was an asymptomatic carrier of typhoid fever. She worked as a cook for several families in New York City at the beginning of the twentieth century and she also cooked for the soldiers. Several cases of typhoid fever in members of those families were traced to her by the Health Department. It appeared that she "carried" the infectious agent without becoming sick. At the time, there was no way of eradicating the disease. Because typhoid is spread primarily through fecal-oral transmission, most of Mary Mallon's transmission risk was thought to arise from her continued involvement in occupations involving food preparation and handling. New York City's public health officials initially sought to merely restrict her from such employment rather than permanently quarantining her. When she continued to be non-compliant, the Health Commission ordered that she be quarantined on one of the islands surrounding Manhattan. She remained there until her death.
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