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There is no evidence that viruses possess a common ancestor or are in any way phylogenetically related. Nevertheless, classification along the lines of the Linnean system into families, genera, and species has been utilized. Based on the organisms they infect, the first broad division of viruses is into bacterial, plant, and animal viruses. Within these classes, other criteria for subdivision are used. Among these are general morphology; envelope or the lack of it; nature of the genome (DNA or RNA); structure of the genome (single- or double-stranded, linear or circular, fragmented or nonfragmented); mechanisms of gene expression and virus replication (positive- or negative-strand RNA); serological relationship; host and tissue susceptibility; pathology (symptoms, type of disease).
The families of animal viruses are sometimes subdivided into subfamilies; the suffix -virinae may then be used. The subgroups of a family or subfamily are equivalent to the genera of the Linnean classification. See Animal virus
The animal DNA viruses are divided into five families: Poxviridae, Herpesviridae, Adenoviridae, Papovaviridae, and Parvoviridae. RNA animal viruses may be either single-stranded or double-stranded. The single-stranded are further subdivided into positive-strand and negative-strand RNA viruses, depending on whether the RNA contains the messenger RNA (mRNA) nucleotide sequence or its complement, respectively. Further, the RNA genes may be located on one or several RNA molecules (nonfragmented or fragmented genomes, respectively). The positive-strand RNA animal viruses contain six families: Picornaviridae, Calciviridae, Coronaviridae, Togaviridae, Retroviridae, and Nodamuraviridae. The nucleocapsid of negative-strand RNA animal viruses contains an RNA-dependent RNA polymerase required for the transcription of the negative strand into the positive mRNAs. Virion RNA is neither capped nor polyadenylated. The group is divided into five families: Arenaviridae, Orthomyxoviridae, Paramyxoviridae, Rhabdoviridae, and Bunyaviridae. The double-stranded RNA animal viruses contain only one group, the Reoviridae.
Bacterial viruses are also known as bacteriophages or phages. They may be tailed or nontailed. Nontailed phages are further subdivided into those with envelopes and those without. Tailed phages, which do not have envelopes, are divided into three families: Myoviridae, Styloviridae, and Pedoviridae. The group of nontailed DNA bacteriophages contains seven families, each with a distinctive morphology: Tectiviridae, Corticoviridae, Inoviridae, Microviridae, Leviviridae, Plasmaviridae, and Cystoviridae. Only the latter two families have envelopes. See Bacteriophage
Plant viruses are divided into groups, rather than families, except those which belong to families of rhabdo viridae and reoviridae. The group, and correspondingly subgroup and type, can be viewed as analogous to family, genus, and species, respectively. Most common among plant viruses are those with a single-stranded, capped but not polyadenylated, positive-strand RNA. See Plant viruses and viroids