Virus classification is based on structural, physico-chemical and replication properties. Some of these criteria have been used since the earliest days of virology (e.g. antigenic relationships) and others have been developed only recently (e.g. nucleic acid sequencing). The taxa in. virology are order, family, genus and species. Each taxonomic level is defined by a specific subset of properties. For example, virion morphology and size, and genome type and stranded-ness are high-level properties and define taxa at the order and/or family level, whereas nucleotide sequence and protein properties are low-level properties and define taxa at the genus and/or species level.
In practice, a virus strain (i.e. the progeny virus from a single isolation) is usually classified from the 'top down'. Classification generally begins with data on virion morphology and the nature of the genome to determine the order and/or family. Additional data are then used to determine the genus, and finally whether the strain is a new species. However, in
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