Abstract: Human immunodeficiency virus (HIV) is the primary etiologic agent for the acquired immune deficiency syndrome (AIDS). HIV exhibits considerable genotypic and phenotypic variability as manifested by different replicative kinetics, susceptibility to serum neutralization, antiviral drug resistance, cytopathic effects, and host-cell range specificity. Infection by HIV leads to progressive deterioration of cell-mediated immune responses making infected patients susceptible to a variety of opportunistic infections, such as pneumocystis carinii pneumonia (PCP), as well as, to the development of tumors such as Kaposi's sarcoma (KS). While the use of live-attenuated viruses to treat HIV infection has been proposed, to date researchers have been unable to demonstrate their efficacy in a clinical setting.