他在朋友遇到困难时趁机提出不公平的交易，这种投机取巧的行为令人不齿。

He made an unfair deal when his friend was in trouble, showing his ___opportunistic behavior.

免疫系统受损的患者容易受到各种机会性感染。

Patients with weakened immune systems are vulnerable to various ___opportunistic infections.