What is HIV? What is AIDS?

HIV stands for Human Immunodeficiency Virus. HIV is a virus that attacks the immune system of people it infects.

AIDS stands for Acquired Immune Deficiency Syndrome. AIDS is a syndrome, meaning it is a group of health problems that together make up a disease.

Being HIV-positive is not the same as having AIDS. Many people feel healthy for many years after being infected by HIV. They might not even know they are infected. These people can still transmit the virus to other people.

According to the U.S. government’s Centers for Disease Control and Prevention (CDC), people are diagnosed with having AIDS if they are HIV-positive and have:

  • A CD4 count of fewer than 200 or less than 14%
    OR
  • An opportunistic infection, condition, or disease

Read the CDC’s definition of AIDS


Watch our Video on What is HIV? What is AIDS?


Learn More From Our Favorite Websites

  • For a good short introduction, see AIDSInfoNet.org’s factsheet What is AIDS?
  • For more detail, see the National Library of Medicine’s webpage on HIV and their page on AIDS.
  • For multimedia, see the National Library of Medicine’s interactive tutorial on HIV and AIDS.
  • For an introduction aimed at youth, see KidsHealth.org’s introduction to HIV and AIDS.
  • Those pages are all available in Spanish as well. For information on HIV/AIDS in many more languages, see our HIV Factsheets page, which has a section on HIV Information in Languages Other Than English.
  • For the official legal definitions of HIV and AIDS, see the CDC’s Case Definition of AIDS. For a more reader-friendly version of this, with a list of the opportunistic infections that can define AIDS, see AIDSmeds.com’s webpage on Opportunistic Infections.

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Every year, we publish a Greater Philadelphia AIDS Resource Guide. The Resource Guide is full of useful organizations in the greater Philadelphia area.

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