An infection that occurs more easily, becomes more severe, or causes disease in a person whose immune defenses are weakened.
A healthy immune system normally keeps many microorganisms under control and protects against infections that would otherwise be difficult to establish. When immune function is significantly impaired, these organisms may cause disease, or infections that are usually mild may become unusually severe.
Opportunistic infections are particularly important in conditions that cause substantial immunosuppression, including advanced HIV infection. Examples include Pneumocystis jirovecii pneumonia, cryptococcosis, and certain forms of tuberculosis. Some opportunistic infections result from organisms that are commonly present in the environment or can remain in the body without causing illness until immune defenses weaken.
QUICK FACT
An opportunistic infection does not necessarily involve an unusual microorganism. Sometimes it is a familiar organism taking advantage of weakened immune defenses.
COMMONLY CONFUSED WITH
Secondary infection: A secondary infection develops following or in association with another infection or condition; an opportunistic infection specifically refers to infection taking advantage of impaired or weakened host defenses.
SEE ALSO