Human immunodeficiency virus (HIV) is the cause of AIDS. AIDS is a potentially deadly illness that interferes with the body's ability to fight off infection and certain types of cancer. Treatments containing multiple drugs (antiretroviral therapy) have greatly improved outcomes for HIV-infected patients. Regimens that contain powerful drugs called protease inhibitors can be particularly effective. These regimens are known as highly active antiretroviral therapy or HAART. Doctors use blood tests called CD4 count and viral load to determine when to start treatment for HIV infection and to monitor patients who are receiving treatment. CD4 cells are involved in fighting infection; they decrease as the disease advances, so a higher count is better. Viral load is a measure of the amount of HIV virus in the blood. It increases as the disease advances, so a lower load is better. CD4 cell count and viral load predict outcomes in HIV infection, but it has been uncertain whether the values of these tests before treatment or after treatment are best for predicting how patients will respond.