What Are the AIDS Defining Illnesses?

Understanding the CDC Classifications

HIV virus in the bloodstream
HIV virus in the bloodstream. Iam Cuming/Ikon Images/Getty Images

 A person is with HIV said to have AIDS when he or she has one or both of the following:

  • either a CD4 count of less than 200 cells/mL; and/or
  • an AIDS-defining illness.

AIDS-defining illnesses are those diseases classified by the Centers for Disease Control and Prevention (CDC) as being directly associated the deterioration of the immune system as a result of HIV infection. Many of these diseases are seen outside the realm of HIV but are classified as AIDS-defining either because they are more prevalent in HIV-positive people or are rarely seen outside of the scope of immune suppressive disorders.

By and large, AIDS-defining illnesses tend to be more serious — and, in some cases, potentially life-threatening — in people with HIV, affecting multiple organ systems. (Keep in mind that these diseases can also occur in people without HIV, although person would not be classified as having AIDS.)

Opportunistic Infection vs. AIDS-Defining Illnesses

While AIDS-defining illnesses can also be classified as opportunistic infections, the opposite is not necessarily true. Opportunistic infections are often ordinary, harmless pathogens in one's body (like viruses, bacteria and fungi) that only cause disease when immune defenses have been breached.

Many opportunistic infections are not life-threatening and can appear even when a person's CD4 count is high. AIDS-defining most often (but not always) appear during later stage disease when the CD4 count has seriously dropped.

Many of the same opportunistic infections, like herpes simplex virus, are only consider to be AIDS-defining when it disseminates (or spreads) beyond the region or organ where it is typically seen.

List of AIDS-Defining Illnesses

The current list of AIDS-defining illnesses according to the CDC are:

What Can I Do?

If you have HIV, seeing an HIV specialist, starting HIV therapy, and getting your CD4 count and viral load monitored regularly are critical for your health and the prevention of HIV-associated illnesses.


AIDS-defining illnesses are best avoided by starting antiretroviral therapy (ART), ideally at the time of diagnosis, which strengthens the immune function. Even when serious HIV-related illnesses develop, ART should always be started in conjunction with the treatment for the illness itself.

Once started, ART must be continued for a lifetime and taken every day without fail. Inconsistent dosing allows for HIV to replicate, undermining the protective benefit of treatment. If allowed to continue, poor drug adherence can lead to treatment failure.

Learn more about the symptoms of HIV by stage.


Centers for Disease Control and Prevention. (December 5, 2008). "Morbidity and Mortality Weekly Report: AIDS-Defining Conditions." Atlanta, Georgia; accessed January 24, 2017.. 

UCSF HIVInSite. (2011). "Opportunistic Infections and AIDS-Related Cancers." San Francisco, California; accessed October 7, 2015. 

Djawe, K.; Buchacz, K.; Hsu, L., et al. "Mortality Risk After AIDS-Defining Opportunistic Illness Among HIV-Infected Persons—San Francisco, 1981–2012." The Journal of Infectious Diseases. June 3, 2015; 212 (9): 1366-1375.

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