It has been little more than four years since the human immunodeficiency virus (HIV) was definitively associated with acquired immunodeficiency syndrome (AIDS). Although a few holdouts still insist that the true cause of AIDS has not yet been found, an over whelming body of evidence indicates that a retrovirus is responsible. The human retroviruses can be classified into transforming retroviruses (HTLV-I and HTLV-II) and cytopathic retroviruses (HIV-I and HIV-Y). As their name suggests, the cytopathic viruses rapidly lyse cells, but it is now clear that they do not lyse all of the cell types that they infect. Many T4 helper cells, which have an abundance of CD4 receptors on their surface, are destroyed following infection. However, HIV does not kill other cell types, such as macrophages, so readily, but rather persists either productively or latently within them. In this fashion, the virus persists in the host generally for the life of the individual.