The present universal system for virus taxonomy is set arbitrarily at the hierarchical levels of family, genus, and species. The most important criteria for classification of viruses are morphology and physical and chemical nature of viral components. Virus families are designated by terms ending in -viridae. Families represent clusters of genera of viruses with apparently common evolutionary origin; for example, it seems unlikely that the very many similar structural and replicative characteristics of all the many diverse poxviruses could stem from more than one common ancestor. Virus genera are designated by terms ending in -virus. The criteria used to designate genera differ between families, and as more viruses are discovered there will be continuing pressures to use more specific physicochemical or serological differences to create new genera in many families. Virus species have not yet been designated formally. This level in the hierarchy of taxa will come to be regarded as equivalent to the present vernacular usage of the term virus.
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