CD4 and tropical spastic paraparesis: HTLV-I, like HIV, is a retrovirus that infects predominantly CD4+ T cells and is responsible for two very different diseases: an invariably fatal T-cell cancer, Adult T-cell Leukemia/lymphoma (ATLL), and a neurodegenerative disease, tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM).