Term formerly used to describe a subfamily of nononcogenic viruses (family Retroviridae) that includes the slow viruses of sheep (visna virus and maedi virus) and human T-cell lymphotropic viruses, including human immunodeficiency viruses types 1 and 2. The viruses resemble the C-type RNA tumor viruses (Oncovirinae) in many ways, including production of reverse transcriptase. [L. lentus, sluggish, slow]