Normally, white blood cells and antibodies attack and destroy foreign organisms
that enter your body. This response is coordinated by white blood cells known as
CD4 lymphocytes. These lymphocytes are also the main targets of HIV, which
attaches to the cells and then enters them. Once inside, the virus inserts its own
genetic material into the lymphocytes and makes copies of itself.
When the new copies of the virus break out of the host cells and enter the
bloodstream, they search for other cells to attack. In the meantime, the old host
cells and some uninfected CD4 cells die from the effects of the virus. The cycle
repeats itself again and again. In the process, billions of new HIV particles are
produced every day. Eventually, the number of CD4 cells in the body decreases,
leading to severe immune deficiency, which means your body can no longer
effectively fight off viruses and bacteria that cause disease.
How HIV is transmitted
You can become infected with HIV in several ways, including:
•Sexual transmission. You may become infected if you have vaginal, anal or oral sex with an infected partner whose blood, semen or vaginal secretions enter your body.
You can also become infected from shared sexual devices if they're not washed or covered with a condom. The virus is present in the semen or vaginal secretions of someone who's infected and enters your body through small tears that can develop
in the vagina or rectum during sexual activity. If you already have another sexually transmitted disease, you're at much greater risk of contracting HIV. Contrary to what researchers once believed, women who use the spermicide nonoxynol 9 also may be at increased risk. This spermicide irritates the lining of the vagina and may cause tears that allow the virus into the body.
•Transmission through infected blood. In some cases, the virus may be transmitted through blood and blood products that you receive in blood transfusions. Since 1985, American hospitals and blood banks have screened the blood supply for HIV antibodies. This blood testing, along with improvements in donor screening and recruitment practices, has substantially reduced the risk of acquiring HIV through a transfusion.
•Transmission through needle sharing. HIV is easily transmitted through needles
and syringes contaminated with infected blood. Sharing intravenous drug paraphernalia puts you at high risk of HIV and other infectious diseases such as hepatitis. Your risk is greater if you inject drugs frequently and also engage in high-risk sexual behavior. Avoiding the use of injected drugs is the most reliable way to prevent infection.
If that isn't an option, you can reduce your risk by participating in a needle exchange program that allows you to trade used needles and syringes for sterile ones.
•Transmission through accidental needle sticks. Transmission of the virus between HIV-positive people and health care workers through needle sticks is low. Experts put the risk at far less than 1 percent.
•Transmission from mother to child. Each year, nearly 600,000 infants are infected with HIV, either during pregnancy or delivery or through breast-feeding. But if women receive treatment for HIV infection during pregnancy, the risk to their babies is significantly reduced. In the United States, most pregnant women are pre-screened for HIV, and anti- retroviral drugs are readily available. Not so in developing nations, where
women seldom know their HIV status, and treatment is often limited or nonexistent. When medications aren't available, Caesarean section is sometimes recommended instead of vaginal delivery. Other options, such as vaginal disinfection, haven't
•Other methods of transmission. In rare cases, the virus may be transmitted through organ or tissue transplants or unsterilized dental or surgical equipment.
Ways HIV is not transmitted
To become infected with HIV, infected blood, semen or vaginal secretions must enter your body. You can't become infected through ordinary contact — hugging, kissing, dancing or shaking hands — with someone who has HIV or AIDS.