AIDS is a serious impairment in a person's immune system. AIDS is the stage of infection with the human immuno-deficiency virus (HIV). HIV is the virus that causes AIDS, and it is HIV, not AIDS, that is spread through the exchange of certain body fluids -- blood, semen, vaginal secretions, and breast milk. A person who has HIV is diagnosed with AIDS by the onset of specific opportunistic infections.

HIV infects and kills certain types of white blood cells called T-helper cells that help protect the body from infections. People who have HIV are vulnerable to serious illnesses cause by bacteria, fungi, viruses, and protozoa, and to some forms of cancer. These illnesses, usually not a threat to people with normally functioning immune systems, are called "opportunistic" because they need the opportunity of a weakened immune system in order to cause illness. It was the unexpected presence of these opportunistic illnesses in a group of previously healthy young gay men in 1981 that first led to the recognition of AIDS by public health workers. At first, their condition was thought to be related to their sexual orientation, however, we now know that anyone who engages in risky behaviors can get HIV.

HIV is a fragile virus. It cannot multiply or complete its life cycle outside the body; it is not spread through the air. Ordinary bleach and water will destroy HIV. HIV is spread through the direct contact of the blood, semen, vaginal secretions, or breast milk of an infected person with another person. This contact is most often made through sexual intercourse with an infected person or the sharing of unsterilized needles or syringes ("works") with an infected person. An infected mother may transmit HIV to her baby during pregnancy, childbirth, or breastfeeding.