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From The Collaborative International Dictionary of English v.0.48:
opportunistic \opportunistic\ adj. 1. taking immediate advantage, often unethically, of any circumstance of possible benefit; practising opportunism. [WordNet sense 1] Syn: opportunist, timeserving. [WordNet 1.5] 2. (Med.) Causing disease only when the immune system of the host has been weakened, or other damage to the host provides special opportunity for invasion; -- of microorganisms or infectious diseases; as, opportunistic pathogens may infect a burn site; AIDS predisposes a patient to opportunistic infection. [PJC]