-opportunistic-

adj. 1. taking immediate advantage, often unethically, of any circumstance of possible benefit; practising opportunism. [ wns=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 ]