HIV (Human Immunodeficiency Virus) is a virus that affects certain white blood cells—CD4 T cells—that manage human immune system responses. When these blood cells are damaged, it becomes difficult for people to fight off infections or diseases.

HIV can be found in body fluids, including:

  • blood
  • semen
  • vaginal fluids
  • rectal fluids
  • breast milk

HIV is passed from one person to another by:

  • having sex (vaginal, anal, or oral) with a person who has HIV
  • sharing needles with a drug user who has HIV
  • during pregnancy, birth, or breast-feeding if a mother has HIV
  • getting a blood transfusion from a person with HIV

AIDS (Acquired Immunodeficiency Syndrome) It can take years for a person infected with HIV, even without treatment, to reach this stage. Having AIDS means that the virus has weakened the immune system to the point at which the body has a difficult time fighting infections. When someone has one or more of these infections and a low number of T cells, he or she has AIDS.