This study will examine the use of the humanized Mik-Beta-1 (Hu Mik-(SqrRoot) 1) monoclonal antibody in patients with HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Some patients infected with the human T-lymphotropic virus type 1 (HTLV-1) virus develop HAM/TSP, a disease in which the immune response to HTLV-1 becomes directed against the person's own body in what is called an autoimmune response. Hu-Mik-Beta-1 is a genetically engineered antibody that blocks the action of a chemical produced by the body during infection or inflammation called interleukin 15 (IL-15). Blocking IL-15 may prevent the autoimmune response that results in HAM/TSP.