Enigmas and Paradoxes: the Genetic Diversity and Prevalence of the Primate Lentiviruses

In a comparatively few years a previously unknown virus has spread from its animal host to infect more than 40 million people by the end of 2003, causing an estimated 3 million deaths in that year (World Health Organization figures). The size of this human immunodeficiency virus (HIV) epidemic and its associated health, social and economic problems make it imperative that we understand how its two types, HIV- 1 and HIV-2, have evolved from their simian relatives, the primate lentiviruses (PLVs). There are several features of the PLVs that may be considered enigmatic or even paradoxical, and which are relevant to studies of their evolution. These reflect the difference in the natural history of PLV-infected apes and humans compared with PLV-infected monkey species, and the apparent host-dependent evolution of some PLV sequences in spite of the relative ease of transmission between primate species.