Human T cell lymphotropic virus (HTLV) types I and II can be transmitted through breast feeding, sexual contact, blood transfusion, and injecting drug use. HTLV-I is endemic in the Caribbean, Japan, South America, and parts of Africa, with HTLV-II found among some native American groups and rarely in Africa.
In Europe, HTLV-II is particularly associated with injecting drug use. While HTLV infection is life-long, fewer than 5% of those infected develop related disease, with first symptoms occurring several years, and maybe decades, after infection. Clinically, HTLV-I may cause adult T cell lymphoma/leukaemia (ATLL) and/or HTLV-I associated myelopathy/tropical spastic paraparesis (HAM/TSP).