opportunisticadj.1. taking immediate advantage, often unethically, of any circumstance of possible benefit; practising opportunism.()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.()