Human T-lymphotropic virus types I and II

Human T-lymphotropic virus types I (HTLV-I) and II (HTLV-II) are members of a family of four known retroviruses that are oncogenic as opposed to cytopathic. This family includes HTLV-I and -II, bovine leukemia virus, and simian T-cell leukemia virus. The two types of HTLV are closely related, and for more than a decade we have been aware of the presence of these viruses in humans. In the first part of this article I summarize recent epidemiologic and clinical findings related to the presence of HTLV-I and -II in the Americas. In the second part, I discuss how these viruses may regulate themselves and how in turn they might cause leukemia and neurologic disease in humans.