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".

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 T-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 T-cells are low enough to allow that OI to develop, your doctor 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 early years of the AIDS epidemic, OIs caused a lot of sickness and deaths. Once people started taking combination antiviral therapy, however, a lot fewer people got OIs. It's not clear how many people with HIV will get a specific OI.

In women, health problems in the vaginal area may be early signs of HIV. These can include pelvic inflammatory disease and bacterial vaginosis, among others. See fact sheet 610 for more information.

The most common OIs are listed here, along with the disease they usually cause, and the T-cell count when the disease becomes active:

• Candidiasis (Thrush) is a fungal infection of the mouth, throat, or vagina. T-cell range: can occur even with fairly high T-cells.

• Cytomegalovirus (CMV) is a viral infection that causes eye disease that can lead to blindness.T-cell range: under 50.

• Herpes simplex viruses can cause oral herpes (cold sores) or genital herpes. These are fairly common infections, but if you have HIV, the outbreaks can be much more frequent and more severe. They can occur at any T-cell count.

• Malaria is common in the developing world. It is more common and more severe in people with HIV infection.

• Mycobacterium avium complex (MAC or MAI) is a bacterial infection that can cause recurring fevers, general sick feelings, problems with digestion, and serious weight loss. T-cell range: under 75.

• Pneumocystis pneumonia (PCP) is a fungal infection that can cause a fatal pneumonia. T-cell range: under 200.

• Toxoplasmosis (Toxo) is a protozoal infection of the brain. T-cell range: under 100.

• Tuberculosis (TB) is a bacterial infection that attacks the lungs, and can cause meningitis. T-cell range: Everyone with HIV who tests positive for exposure to TB should be treated.

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 anti-HIV drugs.

Click here for more information on antiviral therapy.

For each OI, there are specific drugs, or combinations of drugs, that seem to work best. Strong antiviral drugs can allow a damaged immune system to recover and do a better job of fighting OIs.