AIDS is the advanced stage of HIV infection. The criteria of AIDS include HIV infection with fewer than 200 CD4 positive T cells per cubic millimeter of blood (norm is more than 1,000), and two dozens of opportunistic infections, which rarely affect normal people. Opportunistic means that the infection took opportunity of the weakened immune system. AIDS-related infections are often very severe and caused by unusual bacteria, viruses, fungi, and parasites.

Symptoms of AIDS include:

* Weight loss
* Energy loss
* Diarrhea
* Persistent fever
* Cough
* Shortness of breath
* Seizures
* Painful swallowing
* Confusion
* Forgetfulness
* Vision problems
* Persistent headaches
* Children have severe forms of typically childhood infections of eyes, ears and throat.

AIDS patients often develop Kaposi's sarcoma (round brown, reddish, or purple blots on skin) and cervical cancer (in women).Lymphoma is also a common condition. Kaposi's sarcoma-associated herpes virus, human papillomavirus and Epstein -Barr virus are the causative agents. Without antiretroviral therapy, AIDS patent die in a year commonly.

Antiretroviral therapy gives more than 5 years of live. PCP (Pneumocystis carinii (jiroveci) pneumonia) is a common cause of death in AIDS.

Tuberculosis (TB) in AIDS is often multi-drug resistant. Tuberculosis in AIDS may affect bone marrow, bone, kidney, bladder, guts, liver, lymph nodes, brain and spine. It is also more severe in lungs than usual.

Oesophagitis (inflammation of esophagus) is common. Chronic diarrhea in HIV infection is caused by Salmonella, Shigella, Listeria, Campylobacter Escherichia coli, parasites, which happen in normal people.

Cryptosporidiosis, Microsporidiosis, Mycobacterium avium complex and cytomegalovirus are rarely if ever seen in healthy people. However they are common in HIV. Diarrhea also explains significant body wasting in AIDS. Neural system is often affected by parasite Toxoplasma gondii. It infects brain, eyes and lungs.

JC virus causes progressive multifocal leukoencephalopathy that leads to destruction of brain cell connections. It happens only in severely weakened immune deficiency. HIV Infected brain macrophages produce neurotoxins that lead to dementia and encephalopathy.

Cryptococcus is an opportunistic parasite that causes meningitis (inflammation of brain covering tissue). Fevers, headache, fatigue, vomiting, seizures and confusion often develop.

Citomegalovirus may cause colitis (bowel inflammation) and retinitis (an eye layer inflammation) that leads to blindness. Penicillium fungi also cause opportunistic infections especially in Asia.