opportunistic

from WordNet (r) 3.0 (2006)

opportunistic
    adj 1: taking immediate advantage, often unethically, of any
           circumstance of possible benefit [syn: {opportunist},
           {opportunistic}, {timeserving}]

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]