Human immunodeficiency virus (HIV) infection attacks white blood cells known as CD4 lymphocytes, which coordinate the efforts of the immune system in attacking disease. Normally, white blood cells and antibodies attack and destroy foreign organisms (antigens) in the body. HIV attaches to CD4 lymphocytes and “hijacks” them by inserting its own genetic material into the cells and replicating itself.

Eventually, these copies of the virus enter the bloodstream and begin attacking other cells. When this occurs, the original CD4 lymphocyte that spawned the copy dies. The cycle repeats itself over and over, with more than 10 billion new HIV particles produced every day. The immune system reacts by trying to create vast quantities of new CD4 cells – as many as 2 billion new cells each day – but eventually finds itself overpowered by the virus.

As the virus continues to overwhelm the immune system, the number of CD4 cells in the body decreases. This results in severe immune deficiency, in which the body is left unable to fight off viruses and bacteria. The deficiency then becomes AIDS when the CD4 count is less than 200.

HIV can be transmitted in several ways, including:

• Sexual contact. An infected person can transmit the virus to a sexual partner through semen or vaginal secretions that enter the partner’s body. A woman can be infected by either a man or a woman. Vaginal, anal and oral sex are all potential avenues of transmission. Shared sexual devices can also transmit the virus when the device is not disinfected or covered with a condom.
  
  The virus usually enters the body of a sexual partner through tiny tears in the vagina or rectum. The likelihood of transmission greatly increases in people who already have another sexually transmitted disease.
  
  Women should also be aware that the spermicide nonoxynol-9 may irritate the lining of the vagina, causing tears that can increase the risk of transmission.
  
  
• Mother-to-child transmission. Women can transmit HIV to their children through pregnancy, delivery or breastfeeding. However, this risk declines significantly when mothers receive treatment for their HIV infection. In the United States, pregnant women are routinely screened for HIV infection. Those who test positive are prescribed anti-retroviral drugs.
  
  
• Infected blood. HIV can be transmitted through blood and blood products used in blood transfusions. Blood products include whole blood, packed red blood cells, fresh-frozen plasma and platelets. People who received a blood transfusion between 1977 and April 1985 – especially those who received multiple transfusions, such as those with hemophilia (a disease characterized by delayed clotting of the blood) – had a higher risk for exposure to HIV than those who have received transfusions since. The U.S. blood supply has been screened for HIV since 1985, nearly eliminating the risk of contracting HIV through the blood supply since that time.
  
  
• Tainted needles. HIV is easily transmitted through needles and syringes that have come into contact with infected blood, and then are used by someone who does not yet have the virus. Drug addicts who share intravenous drug paraphernalia are especially at risk for this method of transmission. Healthcare workers and others are also sometimes infected when they are accidentally stuck with a needle that has been in contact with an infected person.
  
  
• Other methods. In rare cases, HIV may be transmitted through organ or tissue transplants or unsterile dental or surgical equipment. There also is a small chance of infection during open-mouth kissing if there are bleeding cuts or sores in the mouth present.