CD4 and tropical spastic paraparesis: While the majority of infected individuals remain asymptomatic, a low percentage (2–5%) develop one of two major diseases after a long period of clinical latency: Adult T-cell leukemia/lymphoma (ATLL), a disease characterized by malignant proliferation of CD4+ T-lymphocytes, or HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a neurodegenerative condition [15–18].