Although a member of the lentivirus group, the equine infectious anemia virus (EIAV) was the first nonplant virus discovered in the first decade of the 20th century (1), lentiviruses were considered as rather mysterious viruses until the isolation of the human immunodeficiency virus type 1 (HIV-1) occurred at the beginning of 1980s. Lentiviruses are enveloped viruses carrying two copies of single-strand positive (i.e., codifying) RNA and are considered the ethiologic agents of acquired immunodeficiency syndromes for a broad range of animal species, such as humans, primates, cats, horses, sheep, and goats. Such syndromes develop in multiorgan diseases and share a long period of incubation (with viral persistence despite a potent immunological response) and a fatal outcome. The name lentiviruses (from Latin, lenti, slow) originated from the uniquely prolonged incubation period (i.e., from months to years) needed for the infecting virus to induce the disease, a feature joining the most popular lentivirus, HIV-1, with a large number of nonprimates lentiviruses. Lentiviruses belong to the Lentiviridae subfamily of the Retroviridae family, which also includes the Oncoviridae, for the most part viruses inducing cell transformation, and the Spumaviridae, viruses establishing persistent as well as nonpathogenic infections (a deeper treatment of this topic can be found in ref. 2).