Causes
The virus can be found in the blood (or body fluids containing blood), semen, vaginal fluid, and breast milk of infected people. HIV is also found in saliva, sweat, and tears, but not in high enough amounts to transmit the virus to another person. There are no known cases of anyone catching HIV through sneezing, shaking hands, sharing knives and forks, or from toilet seats and mosquito bites.
The 2 most common ways to be infected with HIV in North America are through unprotected sex and sharing needles. HIV may be transmitted through heterosexual or homosexual vaginal, anal, or oral sex. Although the risk of infection is lower with oral sex, it is still important to use protection during oral sex, such as a dental dam (a piece of latex to cover the vagina during oral sex) or a condom. There is a risk, although it is very small, of becoming infected by open-mouth kissing. There may be small cuts and sores (even if you can't see them) in the mouth that could allow infected blood to pass from one person to another. HIV can also be passed on through perinatal infection - mothers who have HIV are at risk of giving the disease to the baby during birth. The risk of perinatal infection is declining with new treatments. Breast-feeding by an infected mother can also transmit HIV.
The proportion of HIV infections caused by intravenous drug use declined from 28.7% in 1998 to 17.5% in the first half of 2003. Blood transfusions were once a common cause of new infections, but that happens rarely now. Blood products used in transfusion have been heat-treated since 1986. The heat kills the virus. New cases due to hospital accidents such as pinpricks are much rarer than people think. A shallow puncture with an infected needle has an infection risk of 0.3%, and that risk can be further reduced by immediately taking antiviral medication.
Once HIV enters the bloodstream, it takes over cells vital to the immune response, known as CD4+ lymphocytes. The virus then inserts its own genes into the cell, turning it into a miniature factory that produces more copies of the virus. Slowly, the amount of virus in the blood goes up and the number of healthy CD4+ cells goes down. The destruction of CD4+ cells interferes with the body's ability to fight off infections and other diseases.