Pneumocystis carinii pneumonia (PCP) is a type of lung infection that can occur in persons infected with the AIDS virus, HIV. Experts recommend that HIV-infected persons who do not have enough of a certain white blood cell (CD4 cells, with counts below 200) take medications that can prevent PCP. The CD4 count provides an indication of how severely the HIV infection has affected a person's immune system. Highly active antiretroviral therapies (HAART) are newer drug regimens that are very effective at treating HIV infection and raising CD4 counts. With HAART, PCP occurs much less frequently than it used to. Many physicians wonder whether it is safe to discontinue PCP-preventive medications if a patient's CD4 counts go above 200 in response to HAART. Stopping PCP-preventive medications is appealing because the pills can cause unwanted side effects and it reduces the number of pills patients with HIV infection need to take.