Similarly to HIV, the enveloped positive single-stranded RNA virus HTLV-1 infects CD4+ T lymphocytes, and it is the causative agents of two diseases: acute T-cell leukaemia/lymphoma (ATLL) and a demyelinating disease known as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [68]. Here, CD4 is linked to tropical spastic paraparesis.