CD4 and tropical spastic paraparesis: HTLV-1 infection is usually asymptomatic; however, it can cause mainly an aggressive and fatal CD4+ T-cell malignancy, the adult T-cell leukemia (ATL), and a neurodegenerative and disabling disorder of central nervous system, HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) in up to 5% of infected individuals.