Although retroviruses were among the earliest viruses discovered,¹,² they were linked to human disease only in the early 1980s,³–⁵ with the study of T-cell lines from patients with the lymphoproliferative disorder adult T-cell leukemia⁶. T cells from these patients were shown to release a retrovirus, later known as human T-cell lymphotropic virus type I (HTLV-I), the first member of a new class of human viruses⁷. This led to the investigation of a number of idiopathic disorders. In 1985, tropical spastic paraparesis, an endemic disease in the West Indies,⁸ was associated with immune reactivity to HTLV-I in . . .