Medical Definition of opportunistic
1: of, relating to, or being a microorganism that is usually harmless but can become pathogenic when the host's resistance to disease is impaired
2: of, relating to, or being an infection or disease caused by an opportunistic organism <Pneumocystis carinii pneumonia and other opportunistic infections that kill AIDS patients—New York Times>
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Dictionary: Definition of "opportunistic" Spanish Central: Translation of "opportunistic" Nglish: Translation of "opportunistic" for Spanish speakers
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