Home/Topics/Infection & Immunity/opportunistic infection Infection & Immunity Infectious Diseases opportunistic infection Listen Save Share Print Meaning of opportunistic infection: An infection caused by an organism that does not normally cause disease. Definition of opportunistic infection An infection caused by an organism that does not normally cause disease. Opportunistic infections occur in people with weakened immune systems. Written and maintained by James Murray · Last updated July 21, 2026 Educational reference, not medical advice. Read the limitations. Keep exploringRelated termsMore in Infection & Immunity → Infection & Immunity infection Invasion and multiplication of a microorganism in or on the body. Read definition → Medications & Pharmacology doxycycline One of the few antibiotics that reaches organisms living inside human cells, which is why tick-borne and atypical infections come back to it. Read definition → Infection & Immunity cellulitis Redness appearing on both legs at once is rarely cellulitis, since the infection almost always settles on one side only. Read definition → Medications & Pharmacology metronidazole The drug arrives inactive and becomes a poison only inside organisms that live without oxygen, which is the source of its narrow aim. Read definition →
Infection & Immunity infection Invasion and multiplication of a microorganism in or on the body. Read definition →
Medications & Pharmacology doxycycline One of the few antibiotics that reaches organisms living inside human cells, which is why tick-borne and atypical infections come back to it. Read definition →
Infection & Immunity cellulitis Redness appearing on both legs at once is rarely cellulitis, since the infection almost always settles on one side only. Read definition →
Medications & Pharmacology metronidazole The drug arrives inactive and becomes a poison only inside organisms that live without oxygen, which is the source of its narrow aim. Read definition →