HIV and AIDS What is AIDS? AIDS (acquired immune deficiency
syndrome) is caused by the human immunodeficiency virus (HIV),
which kills or impairs cells of the immune system and
progressively destroys the body's ability to fight infections
and certain cancers. HIV is most commonly spread by sexual
contact with an infected partner. The term AIDS applies to the
most advanced stages of an HIV infection. Official criteria for
the definition of AIDS are developed by the Centers for Disease
Control and Prevention (CDC), which is responsible for tracking
the spread of AIDS in the United States. The 2008 CDC definition
of AIDS includes all HIV-infected people who have fewer than 200
CD4+ T cells (healthy adults usually have CD4+ T-cell counts of
800 or more.) In addition, the definition includes HIV-infected
people who have been diagnosed with one or more of 26 clinical
conditions (opportunistic infections) that affect people with
advanced HIV disease. According to the CDC, at the end of 2006,
an estimated 1.1 million