August 13, 2012
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 disease 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 phrase "opportunistic infection" is often shortened to "OI."
The rates of OIs have fallen dramatically since the introduction of antiretroviral therapies. However, OIs are still a problem, especially for people who have not been tested for HIV. Many people still show up in hospitals with a serious OI, often pneumocystis pneumonia. This is how they learn they have HIV infection.
You can be infected with an OI, and "test positive" for it, even though you don't have the disease. For example, almost everyone with HIV tests positive for Cytomegalovirus (CMV). But it is very rare for CMV disease to develop unless the CD4 cell count drops below 50, a sign of serious damage to the immune system.
To see if you're infected with an OI, your blood might be tested for antigens (pieces of the germ that causes the OI) or for antibodies (proteins made by the immune system to fight the germs). If the antigens are found, it means you're infected. If the antibodies are found, you've been exposed to the infection. You may have been immunized against the infection, or your immune system may have "cleared" the infection, or you may be infected. If you are infected with a germ that causes an OI, and if your CD4 cells are low enough to allow that OI to develop, your health care provider will look for signs of active disease. These are different for the different OIs.
People who aren't HIV-infected can develop OIs if their immune systems are damaged. For example, many drugs used to treat cancer suppress the immune system. Some people who get cancer treatments can develop OIs.
HIV weakens the immune system so that opportunistic infections can develop. If you are HIV-infected and develop opportunistic infections, you might have AIDS.
In the US, the Center for Disease Control (CDC) is responsible for deciding who has AIDS. The CDC has developed a list of about 24 opportunistic infections. If you have HIV and one or more of these "official" OIs, then you have AIDS. The list is available at www.thebody.com/content/art14002.html.
In the early years of the AIDS epidemic, OIs caused a lot of sickness and deaths. Once people started taking strong antiretroviral therapy (ART), however, a lot fewer people got OIs. It's not clear how many people with HIV will get a specific OI.
The most common OIs are listed here, along with the disease they usually cause, and the CD4 cell count when the disease becomes active:
Most of the germs that cause OIs are quite common, and you may already be carrying several of these infections. You can reduce the risk of new infections by keeping clean and avoiding known sources of the germs that cause OIs.
Even if you're infected with some OIs, you can take medications that will prevent the development of active disease. This is called prophylaxis. The best way to prevent OIs is to take strong ART. See Fact Sheet 403 for more information on ART.
The fact sheets for each OI have more information on avoiding infection or preventing the development of active disease.
For each OI, there are specific drugs, or combinations of drugs, that seem to work best. Refer to the fact sheets for each OI to learn more about how they are treated. The full US guidelines for treating and preventing OIs can be found here.
Strong antiretroviral drugs (ARVs) can allow a damaged immune system to recover and do a better job of fighting OIs. Fact Sheet 481 on immune restoration has more information on this topic.