Having unprotected sex (sex without a latex condom) with a person who is HIV-positive. The virus can be in an infected person's blood, semen, or vaginal fluids and can enter human body through tiny cuts or sores in skin, or in the lining of vagina, penis, rectum or mouth.
Sharing needles and syringes to inject drugs, or sharing drug equipment used in preparing those drugs with someone who has HIV.
Receiving HIV-infected blood or blood products before 1985. Since 1985, all blood used for transfusions in the United States has been tested for HIV and is generally considered safe.
Carrying, giving birth to or breast-feeding a child while HIV positive. 4