Asymptomatic carrier

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 animals, it is thought that a number of species can act as carriers of human disease. In humans, the HIV virus goes through a long latency period, during which the host is asymptomatic. Most people are infected with persistent viruses such as EBV and Cytomegalovirus, which only rarely progresses to a disease state. Infection by the Polio virus also only rarely leads to the disease by which it is known.

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. 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. There was then no way of eradicating the disease, and an attempt was made to restrict her from continuing to work as a cook to avoid spreading it to others.