HIV is transmitted when the virus enters the body, usually by injecting infected cells or semen. There are several possible ways in which the virus can enter.
Most commonly, HIV infection is spread by having sex with an infected partner. The virus can enter the body through the lining of the vagina, vulva, penis, rectum, or mouth during sex.
HIV frequently spreads among injection-drug users who share needles or syringes that are contaminated with blood from an infected person.
Women can transmit HIV to their babies during pregnancy or birth, when infected maternal cells enter the baby's circulation.
HIV can be spread in health-care settings through accidental needle sticks or contact with contaminated fluids.
Very rarely, HIV spreads through transfusion of contaminated blood or blood components. Blood products are now tested to minimize this risk. If tissues or organs from an infected person are transplanted, the recipient may acquire HIV. Donors are now tested for HIV to minimize this risk.
People who already have a sexually transmitted disease, such as syphilis, genital herpes, chlamydial infection, gonorrhea, or bacterial vaginosis, are more likely to acquire HIV infection during sex with an infected partner.
The virus does not spread through casual contact such as preparing food, sharing towels and bedding, or via swimming pools, telephones, or toilet seats. The virus is also unlikely to be spread by contact with saliva, unless it is contaminated with blood.
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