opportunistic infection

Opportunistic infections are infections caused by organisms that usually do not cause disease in a person with a healthy immune system, but can affect people with a poorly functioning or suppressed immune system. They need an "opportunity" to infect a person.
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Opportunistic infection n. An infection by a microorganism that normally does not cause disease but becomes pathogenic when the body's immune system [more]Opportunistic infection - Community and Resources

Noun
1. any infection caused by a microorganism that does not normally cause disease in humans; occurs in persons with abnormally functioning immune systems (as AIDS patients or transplant patients receiving immunosuppressive drugs)
(hypernym) infection
(hyponym) aspergillosis

An infection caused by an organism that does not normally cause disease. Opportunistic infections occur in people with weakened immune systems.