The human T-cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) were both discovered over three decades ago and infect millions people worldwide. HTLV-1 is associated with the adult T-cell leukemia/lymphoma (ATLL) in about 2% of individuals infected, and another 2 to 3% of individuals develop a neurologic disorder called HTLV-associated myelopathy (HAM). HTLV-2 causes HAM in approximately 1 to 2% of infected individuals, but does not cause ATLL. HTLV-1 and HTLV-2 have served as excellent models for the study of the epidemiology and pathogenesis of virus-associated cancers as well as...