A retrovirus that infects only T lymphocytes (T cells). Human T cell lymphotropic virus type 1 (HTLV-1) is transmitted through exposure to contaminated blood, through sexual contact, and from an infected mother to her child at birth or through breastfeeding via breast milk. HTLV-1 is prevalent mostly in Japan, Africa, the Caribbean Islands, and South America. HTLV-1 can cause adult T-cell leukemia/lymphoma, a rare and aggressive cancer of the white blood cells.