CD4 and tropical spastic paraparesis: HTLV-1 is the sole causative agent of adult T-cell leukemia/lymphoma (ATL), a highly aggressive malignancy of CD4+ T-cells (Yoshida et al., 1982), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic neurodegenerative disease of the spinal cord (Gessain et al., 1985; Osame et al., 1986), and a large number of other inflammatory diseases such as dermatitis, uveitis, and conjunctivitis (Martin et al., 2014).