Scientists believe a virus similar to HIV first occurred in some populations of chimps and monkeys in Africa, where they're hunted for food. Contact with an infected monkey's blood during butchering or cooking may have allowed the virus to cross into humans and become HIV.

How does HIV become AIDS?

HIV destroys CD4 cells — a specific type of white blood cell that plays a large role in helping your body fight disease. Your immune system weakens as more CD4 cells are killed. You can have an HIV infection for years before it progresses to AIDS.

People infected with HIV progress to AIDS when their CD4 count falls below 200 or they experience an AIDS-defining complication, such as:

  • Pneumocystis pneumonia
  • Cytomegalovirus
  • Tuberculosis
  • Toxoplasmosis
  • Cryptosporidiosis

How HIV is 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. HIV can't be transmitted through the air, water or via insect bites.

You can become infected with HIV in several ways, including:

  • By having sex. 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. The virus can enter your body through mouth sores or small tears that sometimes develop in the rectum or vagina during sexual activity.
  • From blood transfusions. In some cases, the virus may be transmitted through blood transfusions. American hospitals and blood banks now screen the blood supply for HIV antibodies, so this risk is very small.
  • By sharing needles. HIV can be 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.
  • During pregnancy or delivery or through breast-feeding. Infected mothers can infect their babies. But receiving treatment for HIV infection during pregnancy, mothers significantly lower the risk to their babies.
May. 20, 2014

You Are ... The Campaign for Mayo Clinic

Mayo Clinic is a not-for-profit organization. Make a difference today.