[ op-er-too-nis-tik, -tyoo- ]
/ ˌɒp ər tuˈnɪs tɪk, -tyu- /


adhering to a policy of opportunism; practicing opportunism.
  1. (of a microorganism) causing disease only under certain conditions, as when a person's immune system is impaired.
  2. (of a disease or infection) caused by such an organism: Pneumocystis pneumonia is an opportunistic disease that often strikes victims of AIDS.

Origin of opportunistic

First recorded in 1890–95; opportunist + -ic

Related forms

op·por·tun·is·ti·cal·ly, adverbun·op·por·tun·is·tic, adjective

Can be confused

opportune opportunistic
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for opportunistic

British Dictionary definitions for opportunistic


/ (ˌɒpətjʊˈnɪstɪk) /


of or characterized by opportunism
med (of an infection) caused by any microorganism that is harmless to a healthy person but debilitates a person whose immune system has been weakened by disease or drug treatment

Derived Forms

opportunistically, adverb
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medicine definitions for opportunistic


[ ŏp′ər-tōō-nĭstĭk ]


Of or relating to an organism capable of causing disease only in a host whose resistance is lowered.
Of or relating to a disease caused by an opportunistic organism.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.