As time goes on, if the CD4 count drops below 200, the person is considered to have advanced HIV disease or acquired immunodeficiency syndrome, called AIDS. If appropriate preventive medications are not started, the HIV-infected person is now at risk for Pneumocystis carinii pneumonia (PCP), cryptococcal meningitis, toxoplasmosis ("toxo"), and other severe opportunistic diseases.

If the CD4 count drops below 50, there is also a risk of Mycobacterium avium complex (MAC) and cytomegalovirus (CMV) infections and lymphoma. Most people who die from HIV disease have CD4 counts in this range.

The course of HIV disease in an individual is quite variable. It is influenced by the strain of virus, and the person's genes and state of health prior to contracting HIV. A small percentage of persons with HIV infection maintain a normal CD4 count over many years, even though they do not receive antiretroviral therapy. These are known as long-term nonprogressors.

What Does AIDS Mean?
AIDS stands for acquired immunodeficiency syndrome, which is a designation established by the Centers for Disease Control and Prevention for the more advanced stages of HIV disease. To meet criteria for an AIDS diagnosis, you must have one of the following: 1) current or past history of severe opportunistic disease (for example, PCP, toxo, KS, cervical cancer, MAC, CMV, tuberculosis); 2) HIV dementia (memory impairment) or wasting syndrome (involuntary weight loss); or 3) CD4 count less than 200.