AIDS-defining illnesses are a specific set of infections and cancers that signal HIV has progressed to its most advanced stage, known as Stage 3 HIV or AIDS. The CDC recognizes roughly two dozen of these conditions. A person living with HIV receives an AIDS diagnosis if they develop any one of these illnesses, or if their CD4 cell count drops to 200 cells per cubic millimeter or below, even without symptoms.
These conditions rarely occur in people with healthy immune systems. Their presence in someone with HIV indicates severe immune damage, which is why they serve as diagnostic markers.
The Full List of AIDS-Defining Conditions
The CDC maintains the official list, which includes opportunistic infections, cancers, and other serious conditions. They fall into several broad categories:
- Fungal infections: Pneumocystis pneumonia (PCP), candidiasis of the esophagus or lungs (but not oral thrush), disseminated coccidioidomycosis, extrapulmonary cryptococcosis, disseminated histoplasmosis
- Parasitic infections: Chronic intestinal cryptosporidiosis (lasting more than one month), chronic intestinal isosporiasis, toxoplasmosis of the brain
- Viral infections: Cytomegalovirus (CMV) disease, CMV retinitis with vision loss, chronic herpes simplex ulcers lasting more than one month, progressive multifocal leukoencephalopathy
- Bacterial infections: Disseminated Mycobacterium avium complex (MAC), tuberculosis at any site, recurrent Salmonella blood infections, recurrent pneumonia
- Cancers: Kaposi sarcoma, Burkitt lymphoma, immunoblastic lymphoma, primary brain lymphoma, invasive cervical cancer
- Other conditions: HIV-related encephalopathy, HIV wasting syndrome
Pneumocystis Pneumonia (PCP)
PCP is one of the most well-known AIDS-defining illnesses and was among the first conditions that alerted doctors to the AIDS epidemic in the early 1980s. It is caused by a fungus that healthy immune systems easily keep in check.
In people with HIV, PCP typically develops gradually over days to weeks rather than hitting all at once. The hallmark symptoms are progressive shortness of breath, a dry cough that produces little or no mucus, fever, and chest discomfort. Fever is present in most cases and sometimes is the only noticeable symptom early on. At rest, a physical exam may seem completely normal, but exertion often reveals rapid breathing and crackling sounds in the lungs. A chest X-ray usually shows a distinctive “ground glass” pattern spreading outward from the center of the lungs, though early in the illness the X-ray can look normal.
Kaposi Sarcoma
Kaposi sarcoma is the most common cancer in people with HIV. It causes distinctive brown, purple, or pink lesions on the skin, most often on the arms, legs, neck, head, and sometimes inside the mouth. Beyond skin changes, it can affect the lungs and intestines, and may cause swelling in the legs. Some people also develop tooth pain or tooth loss, unexplained weight loss, night sweats, or fever lasting longer than two weeks.
The lesions themselves are often the first visible sign, and they can be mistaken for bruises or other skin conditions. Unlike most other cancers, Kaposi sarcoma is driven by a specific herpes virus (HHV-8) that takes advantage of a weakened immune system.
Brain Toxoplasmosis
Toxoplasmosis of the brain is caused by a parasite that many people carry without knowing it. In someone with a healthy immune system, the parasite stays dormant. When HIV severely weakens immunity, the parasite can reactivate and cause swelling in the brain.
The most common presentation is a gradual onset of headache, weakness on one side of the body, and sometimes fever. Some people experience seizures or more generalized confusion. Brain imaging typically reveals multiple ring-shaped lesions, often concentrated in structures deep within the brain, surrounded by swelling. Less commonly, it can appear as a single lesion or as diffuse inflammation without clearly defined spots.
Cytomegalovirus Disease
CMV is another virus that most people carry without problems. It becomes dangerous in people with HIV whose CD4 counts have dropped extremely low, typically below 50 cells per cubic millimeter.
The most frequent form is CMV retinitis, an infection of the retina that starts in one eye but, without treatment or immune recovery, usually spreads to both. It can cause permanent vision loss. CMV can also attack the digestive tract, causing colitis with bloody diarrhea, abdominal pain, weight loss, and fever. In the colon, it can occasionally cause perforation, a medical emergency. Less commonly, CMV affects the esophagus (causing painful swallowing) or the nervous system (causing dementia or other neurological problems).
Tuberculosis
Tuberculosis at any site, whether in the lungs or elsewhere in the body, qualifies as an AIDS-defining condition in a person with HIV. This is notable because TB can occur at any CD4 count, not just at the very lowest levels. It is one of the leading causes of death among people living with HIV worldwide.
TB in someone with advanced HIV can look different from typical TB. It is more likely to spread beyond the lungs to lymph nodes, bones, the brain, or other organs. Chest X-rays may not show the classic cavities seen in people without HIV, making diagnosis trickier.
HIV Wasting Syndrome
Wasting syndrome is defined as an involuntary loss of more than 10 percent of body weight, combined with either chronic diarrhea or persistent weakness and fever lasting at least 30 days. Before effective HIV treatment existed, wasting was one of the most visible and feared consequences of AIDS. It results from a combination of factors: the virus itself increases the body’s metabolic demands, opportunistic infections suppress appetite and impair nutrient absorption, and chronic inflammation breaks down muscle tissue.
Conditions With Specific Qualifiers
Not every case of these illnesses automatically counts as AIDS-defining. Several conditions on the list carry important qualifiers. Candidiasis, for example, only qualifies when it involves the esophagus, airways, or lungs. Oral thrush (candidiasis in the mouth) is common in many stages of HIV and does not, by itself, indicate AIDS.
Herpes simplex only qualifies when ulcers persist for more than one month, or when the virus spreads to the airways or esophagus. Short-lived cold sores or genital herpes outbreaks are not AIDS-defining. Similarly, cryptosporidiosis and isosporiasis, both parasitic gut infections, only count when diarrhea lasts longer than one month. Cervical cancer must be invasive, meaning it has spread beyond the surface of the cervix, to qualify.
Mycobacterium avium complex infection qualifies only when it has spread outside the lungs or disseminated throughout the body. A localized lung infection with MAC does not meet the threshold.
How These Illnesses Relate to CD4 Count
Different AIDS-defining illnesses tend to appear at different levels of immune damage. TB can develop at relatively high CD4 counts, sometimes above 200. Kaposi sarcoma and some lymphomas can also appear before the immune system is profoundly depleted. PCP typically appears when CD4 counts fall below 200, which is the threshold for an AIDS diagnosis on its own. The most dangerous infections, like CMV retinitis and disseminated MAC, generally strike when CD4 counts drop below 50.
This pattern matters because it affects how doctors approach prevention. People with HIV whose CD4 counts fall below certain thresholds are often given preventive medications targeting specific infections, such as PCP, before those infections have a chance to develop.
Why the Distinction Still Matters
With modern antiretroviral therapy, many people with HIV maintain healthy immune systems and never develop any of these conditions. But the classification remains important for several reasons. It drives public health surveillance, helping track how well HIV treatment programs are working at a population level. It also affects individual treatment decisions, since someone diagnosed with an AIDS-defining illness needs both treatment for the specific condition and optimization of their HIV therapy. For people who don’t know they have HIV, one of these illnesses may be the first sign that prompts testing. An estimated 1 in 5 AIDS-defining conditions occurs in people who were not previously diagnosed with HIV.