Definition: An infection is said to be "opportunistic" when it takes advantage of the fact that patients are immunocompromised. Opportunistic infections are infections that are pretty easy for a healthy immune system to fight off. Thus, opportunistic infections only appear in people whose immune system is compromised -- by illness, age, or other factors.
Opportunistic infections are caused by organisms that may be present in healthy people but do not normally cause disease in them. However, when these same organisms infect a person who is immunocompromised, their bodies can not fight off the infection and they become sick.
Opportunistic infections are one of the hallmarks of AIDS, but opportunistic infections do not only occur in AIDS patients. They can occur in anyone who is immune deficient.
Examples:
It was the appearance of previously rare diseases -- such as pneumocystis jirovecii pneumonia -- in AIDS patients that helped early scientists understand that a new type of contagious disease was on the rise. A variety of these opportunistic infections, which were rarely seen in the general population, started showing up in populations connected by sexual networks and other infectious ties. This led doctors to look for a pathogen that could be affecting people's immune systems.
