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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.
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.
HIV infection has a long period during which the person is asymptomatic. Although the host may not be experiencing symptoms, the virus can still be passed on to others. It is also possible for the infection to become symptomatic after this latent period. Whether the host is showing symptoms or not, opportunistic infections can take advantage of the weakened immune system and cause further complications.
Epstein-Barr Virus (EBV)
Many carriers are infected with persistent viruses such as Epstein-Barr Virus (EBV), which is a member of the herpes virus family. Studies show that about 95% of adults have antibodies against EBV, which means they were infected with the virus at some point in their life.
Chlamydia, an STD that affects both men and women, can also be asymptomatic in most individuals. Although the infection is not yielding any symptoms, it can still damage the reproductive system. If the infection goes unnoticed for a long time, the infected individual(s) are at risk of developing Pelvic Inflammatory Disease (PID). Like Chlamydia, PID can also be asymptomatic. 
A small number of asymptomatic carriers (referred to as chronic excretors) continue to produce active virus for years (or even decades) after their initial exposure to the oral Sabin vaccine. This increases the difficulty of Poliomyelitis eradication.
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