simian immunodeficiency virus (SIV), a lentivirus that produces an acquired immunodeficiency syndrome–like disease in nonhuman primates. The cytopathological changes caused by SIV are similar to those caused by the human immunodeficiency virus (HIV). SIV also shares with HIV a group of genes lacking in other retroviruses, and animals infected with either virus experience a similar decrease in the number of CD4+ lymphocytes.
