Dictionary:

opportunistic

  (ŏp'ər-tū-nĭs'tĭk, -tyū-)

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Taking immediate advantage, often unethically, of any circumstance of possible benefit.

1. Of or relating to an organism capable of causing disease only in a host whose resistance is lowered.
2. Of or relating to a disease caused by an opportunistic organism.

1. denoting a microorganism which does not ordinarily cause disease but becomes pathogenic under certain circumstances.
2. denoting a disease or infection caused by such an organism.

The adjective has one meaning:

Meaning #1: taking immediate advantage, often unethically, of any circumstance of possible benefit
  Synonyms: opportunist, timeserving