Opportunistic infection

Infection that develops from a pre-existing condition

An opportunistic infection is an infection that occurs most commonly in individuals with an immunodeficiency disorder and acts more severely on those with a weakened immune system. These types of infections are considered serious and can be caused by a variety of pathogens including viruses, bacteria, fungi, and parasites. Under normal conditions, such as in humans with uncompromised immune systems, an opportunistic infection would be less likely to cause significant harm and would typically result in a mild infection or no effect at all.

From Wikipedia, under CC BY-SA. More on occurri.