Paleovirology—Modern Consequences of Ancient Viruses

Within the past century, a number of ‘‘emerging viruses’’ with pathogenic properties, such as HIV-1, SARS-CoV, and several novel reassortments of influenza A, have entered the human population on a large scale. However, novel pathogenic viral infections of humans are not unique to modern history. ‘‘Paleovirology’’ is the study of ancient extinct viruses (called ‘‘paleoviruses’’) and the effects that these agents have had on the evolution of their hosts. Thus far, the study of these viruses has mostly been limited to endogenous retroviruses that can be directly identified from their remnants in host genomes. However, one can infer the existence of other paleoviruses from their evolutionary pressures on host genes. We suggest that selection to survive the pathogenic effects of these viruses has shaped our repertoire of antiviral defenses in ways that impact our resistance or susceptibility to modernday emerging viruses.