HIV is the cause of AIDS, which is a potentially deadly illness that interferes with the body's ability to fight off infection and certain types of cancer. Treatment regimens containing several drugs (called highly active antiretroviral therapy or HAART) have greatly improved outcomes for HIV-infected patients. Unfortunately, people need to have high levels of adherence to the treatment regimens for the drugs to work. Adherence means taking drugs exactly as prescribed and not missing doses. Newer treatment combinations include drugs called nonnucleoside reverse transcriptase inhibitors (NNRTIs). It is unclear whether treatment combinations that include NNRTIs require the same high level of adherence to effectively treat HIV infection as treatment combinations that do not contain these drugs. Studies of treatment combinations that did not include NNRTIs suggest that patients need to take 90% of prescribed doses to effectively suppress HIV infection.