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 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.
In 2021, the ICTV changed the International Code of Virus Classification and Nomenclature to mandate a binomial format for naming new viral species similar to that used for cellular organisms; the names of species coined prior to 2021 are gradually being converted to the new format, a process planned for completion by the end of 2023.
As of 2022, the ICTV taxonomy listed 11,273 named virus species in 2,818 genera, 264 families, 72 orders, 40 classes, 17 phyla, 9 kingdoms and 6 realms. However, the number of named viruses considerably exceeds the number of named virus species since, by contrast to the classification systems used elsewhere in biology, a virus "species" is a collective name for a group of viruses sharing certain common features. Also, the use of the term "kingdom" in virology does not equate to its usage in other biological groups, where it reflects high level groupings that separate completely different kinds of organisms.

Definitions

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:

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.
As at 2021, the ICTV definition of species states: "A species is the lowest taxonomic level in the hierarchy approved by the ICTV. A species is a monophyletic group of MGEs whose properties can be distinguished from those of other species by multiple criteria", with the comment "The criteria by which different species within a genus are distinguished shall be established by the appropriate Study Group. These criteria may include, but are not limited to, natural and experimental host range, cell and tissue tropism, pathogenicity, vector specificity, antigenicity, and the degree of relatedness of their genomes or genes. The criteria used should be published in the relevant section of the ICTV Report and reviewed periodically by the appropriate Study Group."

Below species rank (named viruses/virus strains/isolates)

Many individually named viruses exist at below the rank of virus species. The ICVCN gives the examples of blackeye cowpea mosaic virus and peanut stripe virus, which are both classified in the species Potyvirus phaseovulgaris, which is a member of the genus Potyvirus. As another example, the virus SARS-CoV-1, that causes severe acute respiratory syndrome is different from the virus SARS-CoV-2, the cause of the COVID-19 pandemic, but both are classified within the same virus species, Betacoronavirus pandemicum. As set out in the ICVCN, section 3.4, the names of taxa below the rank of species are not governed by the ICTV; "Naming of such entities is not the responsibility of the ICTV but of international specialty groups. It is the responsibility of ICTV Study Groups to consider how these entities may best be classified into species." Using the example given above, the virus causing the COVID-19 pandemic was given the designation "SARS-CoV-2" by the Coronaviridae Study Group of the International Committee on Taxonomy of Viruses in 2020; in the same publication, this Study Group recommended a naming convention for particular isolates of this virus "resembl the formats used for isolates of avian coronaviruses, filoviruses and influenza virus" in the format virus/host/location/isolate/date, with a cited example as "SARS-CoV-2/human/Wuhan/X1/2019".

ICTV classification

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, following the methods set out in the International Code of Virus Classification and Nomenclature. 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. Integrative genomic analyses, incorporating pangenomics and phylogenomics, have recently suggested that viral family-level diversity is significantly higher than currently formally recognized. For example, research on the 'extended Asfarviridae' clade of giant viruses indicates that these lineages are deeply divergent, supporting their division into multiple distinct families.
Viral classification starts at the level of realm and continues as follows, with the taxonomic suffixes in parentheses:
In parallel to the system of binomial nomenclature adopted in cellular species, the ICTV has recently mandated that new virus species be named using a binomial format, and that pre-existing virus species names be progressively replaced with new names in the binomial format. A mid-2023 review of the status of this changeover stated: "...a large number of proposals renamed existing species for compliance with the recently mandated binomial nomenclature format. As a result, 8,982 out of the current 11,273 species now have binomial names. The process will be concluded in 2023, with the remaining 2,291 species being renamed."
As of 2025, all levels of taxa except subrealm, subkingdom, and subclass are used. Seven realms, one incertae sedis class, 25 incertae sedis families, and two incertae sedis genera are recognized:
Realms:

Adnaviria
Duplodnaviria
Monodnaviria
Riboviria
Ribozyviria
Singelaviria
Varidnaviria

Incertae sedis classes:
Naldaviricetes
Incertae sedis families:

Alphasatellitidae
Ampullaviridae
Avsunviroidae
Bartogtaviriformidae
Bicaudaviridae
Brachygtaviriformidae
Clavaviridae
Eurekaviridae
Fuselloviridae
Globuloviridae
Guttaviridae
Halspiviridae
Huangdiviridae
Itzamnaviridae
Nipumfusiviridae
Obscuriviridae
Ovaliviridae
Plasmaviridae
Polydnaviriformidae
Portogloboviridae
Pospiviroidae
Rhodogtaviriformidae
Spiraviridae
Thaspiviridae
Tolecusatellitidae

Incertae sedis genera:

Dinodnavirus
''Rhizidiovirus''
Structure-based virus classification

It has been suggested that similarity in virion assembly and structure observed for certain viral groups infecting hosts from different domains of life reflects an evolutionary relationship between these viruses. Therefore, structural relationship between viruses has been suggested to be used as a basis for defining higher-level taxa – structure-based viral lineages – that could complement the ICTV classification scheme of 2010.
The ICTV has gradually added many higher-level taxa using relationships in protein folds. All four realms defined in the 2019 release are defined by the presence of a protein of a certain structural family.

Baltimore classification

Baltimore classification is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid, strandedness, sense, and method of replication. Named after David Baltimore, a Nobel Prize-winning biologist, these groups are designated by Roman numerals. Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar. In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research. Viruses can be placed in one of the seven following groups:

DNA viruses

Viruses with a DNA genome, except for the DNA reverse transcribing viruses, are members of three of the four recognized viral realms: Duplodnaviria, Monodnaviria, and Varidnaviria. But the incertae sedis order Ligamenvirales, and many other incertae sedis families and genera, are also used to classify DNA viruses. The domains Duplodnaviria and Varidnaviria consist of double-stranded DNA viruses; other double-stranded DNA viruses are incertae sedis. The domain Monodnaviria consists of single-stranded DNA viruses that generally encode a HUH endonuclease; other single-stranded DNA viruses are incertae sedis.

Group I: viruses possess double-stranded DNA. Viruses that cause chickenpox and herpes are found here.
Group II: viruses possess single-stranded DNA.

Virus family	Examples	Virion naked/enveloped	Capsid symmetry	Nucleic acid type	Group
1. Adenoviridae	Canine hepatitis virus, Some types of the common cold	Naked	Icosahedral	ds	I
2. Papovaviridae	JC virus, HPV	Naked	Icosahedral	ds circular	I
3. Parvoviridae	Human parvovirus B19, canine parvovirus	Naked	Icosahedral	ss	II
4. Herpesviridae	Herpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein–Barr virus	Enveloped	Icosahedral	ds	I
5. Poxviridae	Smallpox virus, cowpox, myxoma virus, monkeypox, vaccinia virus	Complex coats	Complex	ds	I
6. Anelloviridae	Torque teno virus	Naked	Icosahedral	ss circular	II
	HHPV1, HRPV1	Enveloped		ss/ds linear/circular	I/II