Immunodeficiency viruses are highly species-specific lentiviruses (enveloped, single-stranded RNA) that, as a group, are able to infect many species and cause immune dysfunction. Most notable are feline immunodeficiency virus (FIV), simian immunodeficiency virus (SIV), bovine immunodeficiency virus (BIV), and human immunodeficiency virus (HIV). Most often, CD4 + and CD8 + T cells are infected, but there are variations based on the specific infecting virus and host. Some infected individuals are able to suppress viral replication, but the majority of individuals progressively decline and develop what is referred to in humans as an acquired immunodeficiency syndrome (AIDS). Some primate species are known to be unaffected carriers of their respective SIV strain, and phylogenetic analyses have suggested a long co-evolutionary relationship that supports viral replication without harming the host.