Opportunistic infections are infections in immunodeficient patients caused by pathogens which are incapable of causing infection in immunocompetent individuals.

Opportunistic infections occur in immunodeficiency diseases such as AIDS, as well as in patients who are on immunosuppressant drugs to prevent rejection of organ transplants and patients on some forms of chemotherapy.

Opportunistic pathogens include bacteria, viruses, fungi and protozoa. Typical opportunistic infections include Pneumocystis jiroveci pneumonia, cytomegalovirus infections and aspergillosis. None of these cause significant illness in immunocompetent individuals, but all can be life-threatening in the immunodeficient.

Certain otherwise rare, virally-mediated tumours such as Kaposi's sarcoma could be considered a form of opportunistic infection.