op·por·tu·nist·ic adj \-t(y)ü-ˈnis-tik\
: of, relating to, or being a microorganism that is usually harmless but can become pathogenic when the host's resistance to disease is impaired
: 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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