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The Collaborative International Dictionary of English v.0.48 (gcide)

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]