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.

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. Richard Gilbert was another prolific asymptomatic carrier.

Typhoid Mary's outward appearance may have looked perfectly healthy, but she ended up being responsible for the infection of about 50 people before she was quarantined. Scientists calculate that between 1% and 6% of individuals infected with Salmonella typhi become chronic, asymptomatic carriers like Mary.

Dr. Denise Monack and others set out to discover just how the bacterium that causes typhoid fever can remain dormant in humans after infection. To do this, she developed a mouse model. Unfortunately, S. typhi can only infect humans, but S. typhimurium, a closely related strand, can infect both mice and humans. It was discovered the infection can last in the system of mice for almost their entire lifetimes. The bacterium takes refuge in macrophages, which are inflammatory attack cells that digest invading pathogens. However, by examining the gut lymph nodes of the mice, 42 days after infecting them, it was determined the S. typhimurium actually changes the inflammatory response of the macrophages. Instead of eliciting an inflammatory response from the attack cells, the bacterium switches them into an anti-inflammatory mode, allowing it to survive within the macrophage.

Typhoid fever is an ailment caused by bacterium Salmonella enterica ser. Typhi. An individual can acquire this infection from consuming risky foods or drinks, or by consuming foods or drinks prepared by an infected individual. (Hence, Typhoid Mary). Those who recover from this infection can still carry the bacteria in their cells, and therefore be asymptomatic.

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