1- Acquired immune deficiency syndrome; a cluster of disorders such as Kaposi’s Sarcoma (KS) and opportunistic infections to which the subject is abnormally vulnerable because of collapse of the immune defense system. The cause is a retrovirus, human T-lymphotropic virus type III (HTLV-III) or HIV (human immunodeficiency virus) which infects and suppresses the T4 lymphocyte, the focal cell of the immune system. It also directly attacks specific types of cells in the central nervous system (CNS) and lungs (and perhaps other tissues as well). There is no known cure, and the mortality rate is high. Over 40% of AIDS patients develop neurologic complications at some point in their illness. The most common CNS dysfunction is a generalized encephalopathy or progressive multifocal leukoencephalopathy (PML) that includes dementia as a dominant feature. Less commonly, the dysfunction is due to well-defined focal lesions, including opportunistic infection by Toxoplasma gondii, which may invade nervous tissue and give rise to seizures or more subtle alterations in mentation and behavior. Cases with a presenting picture of acute psychosis without dementia have also been described. Myelopathy and peripheral neuropathy are other neurologic complications.
2- A fatal disease transmitted by transfer of the human immunodeficiency virus, usually during sexual relations or by using needles previously infected by an HIV-positive person; it compromises the person's immune system to such a degree that he or she ultimately dies from cancer or one of any number of infections.