opportunistic infection

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An infection by a microorganism or virus that normally does not cause disease but becomes pathogenic when the body's immune system is impaired, as in AIDS patients.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun medicine Any infection that causes disease and occurs only when the host's immune system is impaired.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any infection caused by a microorganism that does not normally cause disease in humans; occurs in persons with abnormally functioning immune systems (as AIDS patients or transplant patients receiving immunosuppressive drugs)