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.
- 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.
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