virus classification virus classification virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms. viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. the formal taxonomic classification of viruses is the responsibility of the international committee on taxonomy of viruses (ictv) system, although the baltimore classification system can be used to place viruses into one of seven groups based on their manner of mrna synthesis. specific naming conventions and further classification guidelines are set out by the ictv. a catalogue of all the world's known viruses has been proposed and, in 2013, some preliminary efforts were underway. definitions species definition species form the basis for any biological classification system. before 1982, it was thought that viruses could not be made to fit ernst mayr's reproductive concept of species, and so were not amenable to such treatment. in 1982, the ictv started to define a species as "a cluster of strains" with unique identifying qualities. in 1991, the more specific principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted. in july 2013, the ictv definition of species changed to state: "a species is a monophyletic group of viruses whose properties can be distinguished from those of other species by multiple criteria." these criteria include the structure of the capsid, the existence of an envelope, the gene expression program for its proteins, host range, pathogenicity, and most importantly genetic sequence similarity and phylogenetic relationship. the actual criteria used vary by the taxon, and can be inconsistent (arbitrary similarity thresholds) or unrelated to lineage (geography) at times. the matter is, for many, not yet settled. virus definition the currently accepted and formal definition of a 'virus' was accepted by the ictv executive committee in november 2020 and ratified in march 2021, and is as follows: viruses sensu stricto are defined operationally by the ictv as a type of mge (mobile genetic element) that encodes at least one protein that is a major component of the virion encasing the nucleic acid of the respective mge and therefore the gene encoding the major virion protein itself or mges that are clearly demonstrable to be members of a line of evolutionary descent of such major virion protein-encoding entities. any monophyletic group of mges that originates from a virion protein-encoding ancestor should be classified as a group of viruses. ictv classification comparison 1991 and 2018b virus taxonomy by ictv the international committee on taxonomy of viruses began to devise and implement rules for the naming and classification of viruses early in the 1970s, an effort that continues to the present. the ictv is the only body charged by the international union of microbiological societies with the task of developing, refining, and maintaining a universal virus taxonomy. the system shares many features with the classification system of cellular organisms, such as taxon structure. however, some differences exist, such as the universal use of italics for all taxonomic names, unlike in the international code of nomenclature for algae, fungi, and plants and international code of zoological nomenclature. viral classification starts at the level of realm and continues as follows, with the taxonomic suffixes in parentheses: realm (-viria) subrealm (-vira) kingdom (-virae) subkingdom (-virites) phylum (-viricota) subphylum (-viricotina) class (-viricetes) subclass (-viricetidae) order (-virales) suborder (-virineae) family (-viridae) subfamily (-virinae) genus (-virus) subgenus (-virus) species unlike the system of binomial nomenclature adopted in cellular species, there is currently no standardized form for virus species names. at present, the ictv mandates that a species name must contain as few words as possible while remaining distinct, and must not only contain the word virus and the host name. species names often take the form of virus, particularly for higher plants and animals. in 2019, the ictv published a proposal to adopt a more formalized system of binomial nomenclature for virus species names, to be voted on in 2020. however, some virologists later objected to the potential naming system change, arguing that the debate came while many in the field were preoccupied due to the covid-19 pandemic. as of 2023, all levels of taxa except subrealm, subkingdom, and subclass are used. six realms, one incertae sedis class, 22 incertae sedis families, and two incertae sedis genera are recognized: realms: duplodnaviria, monodnaviria, adnaviria, ribozyviria, riboviria, and varidnaviria incertae sedis families and classes: alphasatellitidae ampullaviridae anelloviridae avsunviroidae bartogtaviriformidae bicaudaviridae brachygtaviriformidae clavaviridae fuselloviridae globuloviridae guttaviridae halspiviridae itzamnaviridae naldaviricetes ovaliviridae plasmaviridae polydnaviriformidae portogloboviridae pospiviroidae rhodogtaviriformidae spiraviridae thaspiviridae tolecusatellitidae incertae sedis genera: dinodnavirus, rhizidiovirus structure-based virus classification