Of the first four identified human retroviruses, the human T lymphotropic virus type 2 (HTLV-2) poses an enigma. HTLV-2, also designated as human T leukemia virus based on its initial isolation from a patient reportedly diagnosed with hairy T cell leukemia, has never conclusively been shown to cause cancer, AIDS, or leukemia. In fact, HTLV-2 appears to be nearly non-pathogenic. Nevertheless, the virus causes concern because of its world-wide prevalence among blood donors and its antigenic similarity to HTLV-1, a virus that can cause a fatal T cell leukemia and degenerative neurological disease. Unlike HTLV-1, HTLV-2 lacks these characteristics despite its ability to establish long-term, asymptomatic infection within nearly all infected individuals. Low levels of viremia escape immune surveillance. In fact, HTLV-2 may induce antiviral effects within immune system cells, resulting in protective effects against viruses, including HIV-1. At most, it is reported that the virus may exacerbate common infections among infected individuals with less than a handful of patients manifesting more severe neurologic manifestations. Therefore, authors suggest that HTLV-2 should no longer be regarded as a pathogenic retrovirus, but rather a benign retrovirus adapted to humans due to its longevity and survival throughout thousands of years of persistence with minimal genomic evolution.