AIDS (acquired immune deficiency syndrome) is caused by HIV, the human immunodeficiency virus, which kills or impairs cells of the immune system and progressively destroys the body's ability to fight infections and certain cancers. HIV is most commonly spread by sexual contact with an infected partner. 

The term AIDS applies to the most advanced stages of an HIV infection. Official criteria for the definition of AIDS are developed by the Centers for Disease Control and Prevention (CDC), which is responsible for tracking the spread of AIDS in the United States. The 2008 CDC definition of AIDS includes all HIV-infected people who have fewer than 200 CD4+ T cells (healthy adults usually have CD4+ T-cell counts of 800 or more). In addition, the definition includes HIV-infected people who have been diagnosed with one or more of 26 clinical conditions (opportunistic infections) that affect people with advanced HIV disease.

According to the CDC, at the end of 2011, an estimated 1.2 million adults and adolescents were living with HIV/AIDS in the United States. There are about 50,000 new cases each year. According to the World Health Organization (WHO), at the end of 2008, there were 33.4 million people living with AIDS globally.

“The disease can infect anyone regardless of race, income or sexual preference, and there is still no cure,” says infectious disease specialist Timothy Friel, MD, of Lehigh Valley Health Network.

The virus enters the body through the lining of the vagina, vulva, penis, rectum or mouth during sexual activity. While homosexual sex remains the leading cause of infection in the United States, more than 90 percent of HIV cases in adults and adolescents worldwide are the result of heterosexual contact.

HIV also can be spread through blood, commonly on needles shared to inject drugs or steroids, or for tattoos or piercing. Due to the screening of blood for evidence of HIV infection, the risk of acquiring HIV from blood transfusions is extremely low. HIV also can be spread to babies born to, or breastfed by, mothers infected with the virus.

Because of the changing epidemiology of HIV infection, the CDC now recommends routine HIV testing for all Americans between ages 13 and 64.

HIV can’t be passed by hugging, kissing or donating blood; sharing food, telephones, swimming pools, toilet seats or eating utensils; or through tears, saliva, sweat, urine or insects like mosquitoes. 

To protect yourself, choose a lifestyle that keeps you at low risk for HIV. In general:

  • Use a latex condom when having sex (anal, vaginal or oral) unless you and your partner are not infected and are in a monogamous relationship. Remember that condoms can break, so they do not guarantee your safety. If you have a latex allergy, please notify your health care provider to discuss alternatives.
  • Never share needles or other objects such as razors that have been exposed to another person's blood.
  • When in doubt, ask your partner to be tested for HIV and get tested yourself.