A form of blood cancer affecting the T-cells which make up the body's immune system. The disease is caused by the HTLV-1 virus (human T-cell leukemia virus) which causes the proliferation of abnormal T-cells. The virus can be transmitted sexually and may lay dormant for decades. There are four subtypes: acute, chronic, lymphoma and smoldering. The acute and lymphoma subtypes have the poorest prognosis. See also Adult T-Cell leukemia