Evolution and Origin of Human Viruses

Like all living organisms, humans are constantly exposed to a huge diversity of viruses. Human viruses are genetically highly diverse, and new genotypes, strains and species can evolve rapidly. Ribonucleic acid (RNA) viruses comprise a major part of the eukaryotic virome, whereas few prokaryotic RNA viruses are known. Extant human-infecting RNA viruses are highly diverse in terms of genome size and organization, as well as of infection cycle and transmission dynamics. Similarly to RNA viruses, deoxyribonucleic acid (DNA) viruses display an extraordinary diversity of genome architectures and sizes, replication mechanisms, virion structures and host tropisms. Their genomes may consist of double-stranded DNA (dsDNA) or single-stranded DNA (ssDNA). Extant DNA viruses include many important mammalian and human pathogens, such as herpesviruses, poxviruses, polyoma/papillomavirus and polyomaviruses. All herpesviruses that infect humans belong to the Herpesviridaefamily. The huge biological diversity that characterizes viruses suggests the presence of different strategies to generate genetic change.