A cquired immunodeficiency syndrome (AIDS) Awas first reported in 1981 in'homosexual men. AIDS is a retroviral disease caused by human immunodeficiency virus (HlV). The disease is characterized by immunosuppression, secondary neoplasma and neurological manifestations. AIDS is invariably fatal since there is no cure. In the USA, it is the fourth leading cause of death in men between the ages 15 to 55 years. No other disease has attracted as much aftention as AIDS by the governments, public and scientists. AIDS has stimulated an unprecedented amount of biomedical research which led to a major understanding ofthis deadfy disease within a short period of time. So rapid is the research on AIDS (particularly relating to molecular biofogy), any review is destined to be out of date by the time it is published! The isolation of human immunodeficiency virus (HIV) from lymphocytes of AIDS patients was independently achieved by Gallo (USA) and Montagnier (France) in 1984. Epidemiology AIDS was first described in USA and this country has the majority of reported cases. The prevalence of AIDS has been reported from almost every country. The number of people living with HfV worldwide is estimated to be around 40 million by the end of the year 2005. (lndia alone has about 5 million persons). At least 5 million deaths occurred in 2005, due to AIDS. AIDS is truely a global disease with an alarming increase in almost every country. Transmission of HIV : Transmission of AIDS essentially requires the exchange of body fluids (semen, vaginal secretions, blood, milk) containing the virus or virus-infected celfs. There are three major routes of HIV transmission- sexual contact, parenteral inoculation, and from infected mothers to their newborns. The distribution of risk factors for AIDS trans- mission are as follows. Sex between men (homosexuals) Sex between men and women - 60"/" -15%Intravenous drug abusers - 15"/" Transfusion of blood and blood products - 6% All others - 4o/o The predominant methods of HIV transmission (about 75o/") are through anal or vaginal intercourse. The risk for the transmission is much higher with anal than with vaginal intercourse. The practice of 'needle sharing' is mainly responsible for the transmission of HIV in drug abusers. Pediatric AIDS is mostly caused by vertical transmission (mother to infant). It should, however, be noted that HIV cannot be transmitted by casual personal contact in the household or work place. Further, the transmission of AIDS from an infected individual to health personnel attending on him is extremelv rAre. Virology of HIV AIDS is caused by a retrovirus, namely human immunodeficiency virus (HlY), belonging to lentivirus family. Retroviruses contain RNA as the genetic material. On entry into the host cell, they transcribe DNA which is a complementary copy of RNA. The DNA, in turn is used, as a template to produce new viral RNA copies. Two different forms of HlV, namely HIV-I and HIV-2 have been isolated from AIDS patients. HIV-1 is more common, being found in AIDS patients of USA, Canada, Europe and Central Africa while HIV-2 is mainly found in West Africa. Both the viruses are almost similar except they differ in certain immunological properties. HIV-1 is described in some detail. Structure of HIV : The viruse is spherical with a diameter of about 110 nm. lt contains a core, surrounded by a lipid envelop derived from the host pfasma membrane (Fig.3fl.l). The core of the HIV has two strands of genomic RNA and four core proteins, PZq, PtB, reverse tranScriptase (poolpsr) and endonuclease (p32). Note that the naming of the proteins is based on the molecular weight. For instance, a protein with a molecular weight of 24,0OO is designated as p2,4. The lipid membrane of the virus is studded with two glycoproteins Bprzo and gpot. The surface antigen 8p126 is very important for the viral infection CD4.