A single-stranded human-infecting RNA retrovirus from the subfamily Oncovirinae which causes adult T-cell leukemia and T-cell lymphoma and may also be involved in certain demyelinating diseases (diseases where the protective myelin sheath around nerve fibers is destroyed).
There are two known types, HTLV-I and HTLV-II. The two viruses are closely related to each other (60% of their genomes are identical) but the type and severity of the disease they cause may differ.
"Human T-lymphotropic virus type III" (HTLV-III) is an obsolete term for human immunodeficiency virus (HIV). Some references to HTLV-III may be found in medical literature of the early 1980s
This writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/