In our bodies, we carry many germs -- bacteria, protozoa, fungi, and viruses. When our immune system is working, it controls these germs. But when the immune system is weakened by HIV or by some medications, these germs can get out of control and cause health problems.
Infections that take advantage of weakness in the immune defenses are called "opportunistic."
The rates of Opportunistic infections have fallen dramatically since the introduction of antiretroviral therapies. However, these infections are still a problem, especially for people who have not been tested for HIV.