View source for Virus classification

{{short description|Organisation of viruses into a taxonomic system}}
[[File:Virus classification L Pengo vflip.svg|thumb|Virus classification]]
{{cs1 config|name-list-style=vanc}}
'''Virus classification''' is the process of naming [[virus]]es and placing them into a [[taxonomy (biology)|taxonomic]] system similar to the classification systems used for [[cell (biology)|cellular organisms]].

Viruses are classified by [[phenotypic]] characteristics, such as [[Virus#Structure|morphology]], [[nucleic acid]] type, mode of replication, [[Host (biology)|host organism]]s, 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.

In 2021, the ICTV changed the International Code of Virus Classification and Nomenclature (ICVCN) to mandate a binomial format (genus|| ||species) 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.{{Update needed|date=August 2024|reason=Was this completed by the end of 2023?}}

As of 2022, the ICTV taxonomy listed 11,273 named virus species (including some classed as satellite viruses and others as viroids) in 2,818 genera, 264 families, 72 orders, 40 classes, 17 phyla, 9 kingdoms and 6 realms.<ref name="Virus Taxonomy 2022">{{cite web|url=https://ictv.global/taxonomy|title=Virus Taxonomy: 2022 v3 Release|website=ictv.global|publisher=International Committee on Taxonomy of Viruses|access-date=5 January 2024}}</ref> 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 (presumably related) viruses sharing certain common features (see below). 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 (see [[Kingdom (biology)]]).

==Definitions==
{{see also|Viral quasispecies}}

=== 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:<ref>{{cite journal|last1=Koonin|first1=Eugene V.|last2=Dolja|first2=Valerian V.|last3=Krupovic|first3=Mart|last4=Kuhn|first4=Jens H.|date=2021-09-01|title=Viruses Defined by the Position of the Virosphere within the Replicator Space|journal=Microbiology and Molecular Biology Reviews|volume=85|issue=4|article-number=e0019320|language=EN|doi=10.1128/MMBR.00193-20|pmc=8483706|pmid=34468181}}</ref>

{{blockquote|Viruses ''[[sensu stricto]]'' are defined operationally by the ICTV as a type of [[Mobile genetic elements|MGE]] 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.}}

=== 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 [[wikt:polythetic|polythetic]] class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted.<ref name=alimpiev>{{cite book |last1=Alimpiev |first1=Egor |title=Rethinking the Virus Species Concept |date=March 15, 2019 |url=https://stanford.edu/~alimpiev/thnk_ppr.pdf |archive-url=https://web.archive.org/web/20200922152304/http://stanford.edu/~alimpiev/thnk_ppr.pdf |archive-date=2020-09-22 |url-status=live}}</ref>

As at 2021 (the latest edition of the ICVCN), 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 ([[mobile genetic element]]s) 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."<ref name=ictvcode />

=== Below species rank (named viruses/virus strains/isolates) ===
Many individually named ''viruses'' (sometimes referred to as "virus strains") 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 ''[[Bean common mosaic virus]]'', the latter a member of the genus ''[[Potyvirus]]'' that will in due course receive a binomial name as ''Potyvirus [species...]''. As another example, the virus [[SARS-CoV-1]], that causes severe acute respiratory syndrome ([[SARS]]) is different from the virus [[SARS-CoV-2]], the cause of the [[COVID-19 pandemic]], but both are classified within the same virus species, a member of the genus ''[[Betacoronavirus]]'' that is currently known as ''[[Severe acute respiratory syndrome-related coronavirus]]'' which, per the 2021 mandate from the ICTV, will also receive a binomial name in due course. As set out in the ICVCN, section 3.4, the names [and definitions] 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."<ref name=ictvcode /> Using the example given above, the virus causing the COVID-19 pandemic was given the designation "SARS-CoV-2" by the ''Coronaviridae'' Study Group (CSG) 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[ing] 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".<ref>{{cite journal |author=Coronaviridae Study Group of the International Committee on Taxonomy of Viruses |title=The species Severe acute respiratory syndrome-related coronavirus: classifying 2019-nCoV and naming it SARS-CoV-2 |journal=Nature Microbiology |volume=5 |issue=4 |pages=536–544 |date=April 2020 |pmid=32123347 |pmc=7095448 |doi=10.1038/s41564-020-0695-z}}</ref>

==ICTV classification==
[[File:Comparison 1991 and 2019 virus taxonomy ICTV.webp|thumb|Comparison 1991 and 2018b virus taxonomy by ICTV|upright=1.4]]

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.<ref name=ictvcode>{{cite web|url=https://ictv.global/about/code|title=The International Code of Virus Classification and Nomenclature (ICVCN), March 2021 edition |publisher=International Committee on Taxonomy of Viruses|access-date=6 January 2024}}</ref><ref>{{cite journal | vauthors = Lefkowitz EJ, Dempsey DM, Hendrickson RC, Orton RJ, Siddell SG, Smith DB | title = Virus taxonomy: the database of the International Committee on Taxonomy of Viruses (ICTV) | journal = Nucleic Acids Research | volume = 46 | issue = D1 | pages = D708–D717 | date = January 2018 | pmid = 29040670 | pmc = 5753373 | doi = 10.1093/nar/gkx932 }}</ref> The system shares many features with the classification system of cellular [[organism]]s, 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:<ref name=ictvcode />

:[[Realm (virology)|Realm]] (''-viria'')
::Subrealm (''-vira'')
:::[[Kingdom (biology)#Viruses|Kingdom]] (''-virae'')
::::Subkingdom (''-virites'')
:::::[[Phylum]] (''-viricota'')
::::::Subphylum (''-viricotina'')
:::::::[[Class (biology)|Class]] (''-viricetes'')
::::::::Subclass (''-viricetidae'')
:::::::::[[Order (biology)|Order]] (''-virales'')
::::::::::Suborder (''-virineae'')
:::::::::::[[Family (biology)|Family]] (''-viridae'')
::::::::::::Subfamily (''-virinae'')
:::::::::::::[[Genus]] (''-virus'')
::::::::::::::Subgenus (''-virus'')
:::::::::::::::[[Species]]

In parallel to the system of [[binomial nomenclature]] adopted in cellular species, the ICTV has recently (2021) mandated that new virus species be named using a binomial format (''Genus species'', e.g. ''[[SARS-related coronavirus|Betacoronavirus pandemicum]]''), and that pre-existing virus species names be progressively replaced with new names in the binomial format.<ref>{{cite journal |first1=Peter J. |last1=Walker |first2=Stuart G. |last2=Siddell |first3=Elliot J. |last3=Lefkowitz |display-authors=etal |title = Changes to virus taxonomy and to the International Code of Virus Classification and Nomenclature ratified by the International Committee on Taxonomy of Viruses (2021) | journal = Archives of Virology | volume = 168 | issue = 9| pages = 2633–2648 | date = 6 July 2021 | pmid = 34231026| pmc = | doi = 10.1007/s00705-021-05156-1 |hdl = 10362/134245 |hdl-access = free }}</ref> A mid-2023 review of the status of this changeover stated: "...a large number of proposals [concerning virus nomenclature, submitted to the ICTV Executive Committee (EC) for its consideration] renamed existing species for compliance with the recently mandated binomial nomenclature format. As a result, 8,982 out of the current 11,273 species (80%) now have binomial names. The process will be concluded in 2023, with the remaining 2,291 species being renamed."<ref>{{cite journal |first1=F.M. |last1=Zerbini |first2=S.G. |last2=Siddell |first3=E.J. |last3=Lefkowitz |display-authors=etal |title = Changes to virus taxonomy and the ICTV Statutes ratified by the International Committee on Taxonomy of Viruses (2023) | journal = Archives of Virology | volume = 168 | issue = article no. 175 | date = 10 June 2023 |page=175 | pmid = 37296227| pmc =10861154 | doi = 10.1007/s00705-023-05797-4 }}</ref>

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:<ref name="Virus Taxonomy 2021">{{cite web|url=https://ictv.global/taxonomy|title=Virus Taxonomy: 2024 Release|website=ictv.global|publisher=International Committee on Taxonomy of Viruses|access-date=9 March 2025}}</ref>

'''Realms''': 
* ''[[Adnaviria]]''
* ''[[Duplodnaviria]]''
* ''[[Monodnaviria]]''
* ''[[Riboviria]]''
* ''[[Ribozyviria]]''
* ''[[Singelaviria]]''
* ''[[Varidnaviria]]''
'''''Incertae sedis'' classes''':
* ''[[Naldaviricetes]]''
'''''Incertae sedis'' families''':
{{div col|colwidth=18em}}
* ''[[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]]''

{{div col end}}

'''''Incertae sedis'' genera''': 
* ''[[Dinodnavirus]]''
* ''[[Rhizidiovirus]]''

===Structure-based virus classification===
{{see also|List of higher virus taxa}}
It has been suggested that similarity in virion assembly and structure observed for certain viral groups infecting hosts from different domains of life (e.g., bacterial tectiviruses and eukaryotic adenoviruses or prokaryotic Caudovirales and eukaryotic herpesviruses) reflects an evolutionary relationship between these viruses.<ref>{{cite journal | vauthors = Bamford DH | title = Do viruses form lineages across different domains of life? | journal = Research in Microbiology | volume = 154 | issue = 4 | pages = 231–6 | date = May 2003 | pmid = 12798226 | doi = 10.1016/S0923-2508(03)00065-2 | doi-access = free }}</ref> 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.<ref>{{cite journal | vauthors = Krupovič M, Bamford DH | title = Order to the viral universe | journal = Journal of Virology | volume = 84 | issue = 24 | pages = 12476–9 | date = December 2010 | pmid = 20926569 | pmc = 3004316 | doi = 10.1128/JVI.01489-10 }}</ref>

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.<ref>{{cite web|url=https://ictv.global/taxonomy|title=Virus Taxonomy: 2019 Release|date=|website=talk.ictvonline.org|publisher=International Committee on Taxonomy of Viruses|access-date=26 April 2020|quote=}}</ref>

==Baltimore classification==<!-- This section is linked from [[DNA virus]] -->
{{main|Baltimore classification}}
[[File:Baltimore Classification.svg|thumb|The Baltimore Classification of viruses is based on the method of viral [[mRNA]] synthesis]]
Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their [[nucleic acid]] ([[DNA]] or [[RNA]]), strandedness (single-stranded or double-stranded), [[Sense (molecular biology)|sense]], and method of [[Viral replication|replication]].<ref>{{Cite web |date=2017-06-24 |title=9.3B: The Baltimore Virus Classification |url=https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Boundless)/09:_Viruses/9.03:_Classifying_Viruses/9.3B:_The_Baltimore_Virus_Classification |access-date=2024-09-11 |website=Biology LibreTexts |language=en}}</ref> 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:<ref>{{cite web|title=Baltimore Classification of Viruses|url=http://www.web-books.com/MoBio/Free/Ch1E2.htm|access-date=2023-01-02|website=www.web-books.com}}</ref>

{{Baltimore groups}}
[[File:The Baltimore Classification.gif|left|thumb|Visualization of the 7 groups of virus according to the Baltimore Classification]]
{{clear}}

===DNA viruses===
{{further|DNA virus}}

Viruses with a DNA [[genome]], except for the DNA [[Virus classification#Reverse transcribing viruses|reverse transcribing viruses]], are members of three of the four recognized [[Realm (virology)|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-tag|HUH]] [[endonuclease]]; other single-stranded DNA viruses are ''incertae sedis''.<ref name="Virus Taxonomy 2019">{{cite web|url=https://ictv.global/taxonomy|title=Virus Taxonomy: 2019 Release|website=talk.ictvonline.org|publisher=International Committee on Taxonomy of Viruses|access-date=26 April 2020}}</ref>
* '''Group I''': viruses possess double-stranded DNA. Viruses that cause [[chickenpox]] and [[herpes]] are found here.
* '''Group II''': viruses possess single-stranded DNA.

{| class="wikitable"
|+Examples of DNA viruses
|-
! Virus family
! Examples (common names)
! Virion<br /> naked/enveloped
! Capsid<br />symmetry
! Nucleic&nbsp;acid type
! Group
|-
| 1. ''[[Adenoviridae]]''
| [[Canine hepatitis virus]], Some types of the [[common cold]]
| Naked
| Icosahedral
| ds
| I
|-
| 2. ''[[Papovaviridae]]''
| [[JC virus]], [[Human papillomavirus infection|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]]
| [[Viral envelope|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
|-
| {{nowrap|7. ''[[Pleolipoviridae]]''}}
| [[Haloarcula hispanica pleomorphic virus 1|HHPV1]], [[Halorubrum pleomorphic virus 1|HRPV1]]
| Enveloped
|
| ss/ds linear/circular
| I/II
|}

===RNA viruses===
{{further|RNA virus}}

All viruses that have an [[RNA]] [[genome]], and that encode an [[RNA-dependent RNA polymerase]] (RdRp), are members of the kingdom ''[[Orthornavirae]]'', within the realm ''[[Riboviria]]''.<ref name="ictv">{{cite web|title=Virus Taxonomy: 2019 Release|url=https://ictv.global/taxonomy|website=talk.ictvonline.org|publisher=International Committee on Taxonomy of Viruses|access-date=25 April 2020}}</ref>
* '''Group III''': viruses possess double-stranded RNA genomes, e.g. [[rotavirus]].
* '''Group IV''': viruses possess positive-sense single-stranded RNA genomes. Many well known viruses are found in this group, including the [[picornaviruses]] (which is a family of viruses that includes well-known viruses like Hepatitis A virus, enteroviruses, rhinoviruses, poliovirus, and foot-and-mouth virus), [[Severe Acute Respiratory Syndrome|SARS]] virus, [[hepatitis C]] virus, [[yellow fever]] virus, and [[rubella]] virus.
* '''Group V''': viruses possess negative-sense single-stranded RNA genomes. [[Ebola]] and [[Marburg virus]]es are well known members of this group, along with [[influenza virus]], [[measles]], [[mumps]] and [[rabies]].

{| class="wikitable"
|+Examples of RNA viruses
|-
! Virus Family
! Examples (common names)
! Capsid<br> naked/enveloped
! Capsid<br> Symmetry
! Nucleic&nbsp;acid type
! Group
|-
| 1. ''[[Reoviridae]]''
| [[Reovirus]], [[rotavirus]]
| Naked
| Icosahedral
| ds
| III
|-
| 2. ''[[Picornaviridae]]''
| [[Enterovirus]], [[rhinovirus]], [[hepatovirus]], [[cardiovirus]], [[aphthovirus]], [[poliovirus]], [[parechovirus]], [[erbovirus]], [[kobuvirus]], [[teschovirus]], [[Coxsackievirus|coxsackie]]
| Naked
| Icosahedral
| ss
| IV
|-
| 3. ''[[Caliciviridae]]''
| [[Norovirus|Norwalk virus]]
| Naked
| Icosahedral
| ss
| IV
|-
| 4. ''[[Togaviridae]]''
| [[Eastern equine encephalitis]], [[Chikungunya]]
| [[Viral envelope|Enveloped]]
| Icosahedral
| ss
| IV
|-
| 5. ''[[Arenaviridae]]''
| [[Lymphocytic choriomeningitis virus]], [[Lassa fever]]
| [[Viral envelope|Enveloped]]
| Complex
| ss(−)
| V
|-
| 6. ''[[Flaviviridae]]''
| [[Dengue virus]], [[hepatitis C]] virus, [[yellow fever]] virus, [[Zika virus]]
| [[Viral envelope|Enveloped]]
| Icosahedral
| ss
| IV
|-
| {{nowrap|7. ''[[Orthomyxoviridae]]''}}
| [[Influenzavirus A]], [[influenzavirus B]], [[influenzavirus C]], [[isavirus]], [[thogotovirus]]
| [[Viral envelope|Enveloped]]
| Helical
| ss(−)
| V
|-
| 8. ''[[Paramyxoviridae]]''
| [[Measles virus]], [[mumps virus]], [[respiratory syncytial virus]], [[Rinderpest]] virus, [[canine distemper virus]]
| [[Viral envelope|Enveloped]]
| Helical
| ss(−)
| V
|-
| 9. ''[[Bunyaviridae]]''
| [[California encephalitis virus]], [[Sin nombre virus]]
| [[Viral envelope|Enveloped]]
| Helical
| ss(−)
| V
|-
| 10. ''[[Rhabdoviridae]]''
| [[Rabies virus]], [[Vesicular stomatitis]]
| [[Viral envelope|Enveloped]]
| Helical
| ss(−)
| V
|-
| 11. ''[[Filoviridae]]''
| [[Ebola virus]], [[Marburg virus]]
| [[Viral envelope|Enveloped]]
| Helical
| ss(−)
| V
|-
| 12. ''[[Coronaviridae]]''
|[[Human coronavirus 229E]], [[Human coronavirus NL63]], [[Human coronavirus OC43]], [[Human coronavirus HKU1]], [[Middle East respiratory syndrome-related coronavirus]], [[Severe acute respiratory syndrome coronavirus]], and [[Severe acute respiratory syndrome coronavirus 2]]
| [[Viral envelope|Enveloped]]
| Helical
| ss
| IV
|-
| 13. ''[[Astroviridae]]''
| [[Astrovirus]]
| Naked
| Icosahedral
| ss
| IV
|-
| 14. ''[[Bornaviridae]]''
| [[Borna disease virus]]
| [[Viral envelope|Enveloped]]
| Helical
| ss(−)
| V
|-
| 15. ''[[Arteriviridae]]''
| [[Arterivirus]], [[Equine viral arteritis|equine arteritis virus]]
| [[Viral envelope|Enveloped]]
| Icosahedral
| ss
| IV
|-
| 16. ''[[Hepeviridae]]''
| [[Hepatitis E virus]]
| Naked
| Icosahedral
| ss
| IV
|}

===Reverse transcribing viruses===
All viruses that encode a [[reverse transcriptase]] (also known as RT or RNA-dependent DNA polymerase) are members of the class ''[[Revtraviricetes]]'', within the phylum ''Arterviricota'', kingdom ''Pararnavirae'', and realm ''[[Riboviria]]''. The class ''Blubervirales'' contains the single family ''[[Hepadnaviridae]]'' of DNA RT (reverse transcribing) viruses; all other RT viruses are members of the class [[Ortervirales]].<ref name="Riboviria">{{cite web|title=Proposal: Create a megataxonomic framework, filling all principal taxonomic ranks, for realm Riboviria|url=https://ictv.global/taxonomy/taxondetails?taxnode_id=201907238|vauthors=Koonin EV, Dolja VV, Krupovic M, Varsani A, Wolf YI, Yutin N, Zerbini M, Kuhn JH|website=International Committee on Taxonomy of Viruses (ICTV)|language=en|access-date=2020-05-21}}</ref>
* '''Group VI''': viruses possess single-stranded RNA viruses that replicate through a DNA intermediate. The [[retrovirus]]es are included in this group, of which [[HIV]] is a member.
* '''Group VII''': viruses possess double-stranded DNA genomes and replicate using reverse transcriptase. The [[hepatitis B]] virus can be found in this group.
{| class="wikitable"
|+Examples of reverse transcribing viruses
|-
! Virus Family
! Examples (common names)
! Capsid<br> naked/enveloped
! Capsid<br> Symmetry
! Nucleic&nbsp;acid type
! Group
|-
| 1. ''[[Retrovirus|Retroviridae]]''
| [[HIV]]
| [[Viral envelope|Enveloped]]
|
|dimer RNA
|VI
|-
| 2. ''[[Caulimoviridae]]''
| [[Caulimovirus]], {{nowrap|[[Cacao swollen-shoot virus]] (CSSV)}}
| Naked
|
|
|VII
|-
| {{nowrap|3. ''[[Hepadnaviridae]]''}}
| [[Hepatitis B virus]]
| [[Viral envelope|Enveloped]]
| Icosahedral
| circular, partially ds
|VII
|}

== Historical systems ==

=== Holmes classification ===
[[Francis O. Holmes|Holmes]] (1948) used a [[Linnaean taxonomy]] with [[binomial nomenclature]] to classify viruses into 3 groups under one order, [[Virales]]. They are placed as follows:
* '''Group I:''' ''[[Bacteriophage|Phaginae]]'' (attacks bacteria)
* '''Group II:''' ''[[Plant virus|Phytophaginae]] ''(attacks plants)
* '''Group III:''' ''[[Animal virus|Zoophaginae]]'' (attacks animals)

The system was not accepted by others due to its neglect of morphological similarities.<ref>{{cite book |last1=Kuhn |first1=Jens H. |chapter=Virus Taxonomy |title=Reference Module in Life Sciences |series=Encyclopedia of Virology |date=2020 |pages=28–37 |doi=10.1016/B978-0-12-809633-8.21231-4 |isbn=978-0-12-809633-8 |pmc=7157452 }}</ref>

==Subviral agents==

[[List of subviral agents|Subviral agent]]s are smaller than viruses and have only some of their properties.<ref name="subviral">{{cite web |work=ICTV 9th Report (2011) |title=Subviral Agents |url=https://talk.ictvonline.org/ictv-reports/ictv_9th_report/sub-viral-agents-2011/ |access-date=2020-06-15 |publisher=International Committee on Taxonomy of Viruses |url-status=dead |archive-url=https://web.archive.org/web/20220707080238/https://talk.ictvonline.org/ictv-reports/ictv_9th_report/sub-viral-agents-2011/ |archive-date= Jul 7, 2022 }} Updated version in sync with current release: {{cite web |title=Subviral Agents |url=https://talk.ictvonline.org/ictv-reports/ictv_online_report/subviral-agents/ |url-status=dead |archive-url=https://web.archive.org/web/20220702011937/https://talk.ictvonline.org/ictv-reports/ictv_online_report/subviral-agents/ |archive-date=Jul 2, 2022 |work=10th Report |publisher=International Committee on Taxonomy of Viruses}}</ref><ref name="STRAUSS STRAUSS 2008 pp. 345–368">{{cite book | last1=Strauss | first1=James H. | last2=Strauss | first2=Ellen G. | title=Viruses and Human Disease | chapter=Subviral Agents | publisher=Elsevier | year=2008 | isbn=978-0-12-373741-0 | doi=10.1016/b978-0-12-373741-0.50012-x | pages=345–368| s2cid=80872659 |doi-access=free }}</ref> Since 2015, the ICTV has allowed them to be classified in a similar way as viruses are.<ref>TaxoProp 2015.002aG</ref>

===Viroids and virus-dependent agents===

====Viroids====

{{main|Viroid}}
* Family ''[[Avsunviroidae]]''<ref>{{cite web |last1=Di Serio |first1=Francesco |last2=Li |first2=Shi-Fang |last3=Matoušek |first3=Jaroslav |last4=Owens |first4=Robert A. |last5=Pallás |first5=Vicente |last6=Randles |first6=John W. |last7=Sano |first7=Teruo |last8=Verhoeven |first8=Jacobus Th. J. |last9=Vidalakis |first9=Georgios |last10=Flores |first10=Ricardo |publisher=ICTV |title=Family: ''Avsunviroidae''|url=https://ictv.global/report/chapter/avsunviroidae/avsunviroidae |website=The 10th ICTV Report on Virus Classification and Taxon Nomenclature |date=June 2020 |access-date=16 January 2023}}</ref>
** Genus ''[[Avsunviroid]]''; [[type species]]: ''[[Avocado sunblotch viroid]]''
** Genus ''[[Pelamoviroid]]''; type species: ''[[Peach latent mosaic viroid]]''
** Genus ''[[Elaviroid]]''; type species: ''[[Eggplant latent viroid]]''
* Family ''[[Pospiviroidae]]''<ref>{{cite web |last1=Di Serio |first1=Francesco |last2=Owens |first2=Robert A. |last3=Li |first3=Shi-Fang |last4=Matoušek |first4=Jaroslav |last5=Pallás |first5=Vicente |last6=Randles |first6=John W. |last7=Sano |first7=Teruo |last8=Verhoeven |first8=Jacobus Th. J. |last9=Vidalakis |first9=Georgios |last10=Flores |first10=Ricardo |publisher=ICTV |title=Family: ''Pospiviroidae''|url=https://ictv.global/report/chapter/pospiviroidae/pospiviroidae |website=The 10th ICTV Report on Virus Classification and Taxon Nomenclature |date=November 2020 |access-date=16 January 2023}}</ref>
** Genus ''[[Pospiviroid]]''; type species: ''[[Potato spindle tuber viroid]]''
** Genus ''[[Hostuviroid]]''; type species: ''[[Hop stunt viroid]]''
** Genus ''[[Cocadviroid]]''; type species: ''[[Coconut cadang-cadang viroid]]''
** Genus ''[[Apscaviroid]]''; type species: ''[[Apple scar skin viroid]]''
** Genus ''[[Coleviroid]]''; type species: ''[[Coleus blumei viroid 1]]''

====Satellites====
{{main|Satellite (biology)}}
Satellites depend on co-infection of a host cell with a [[helper virus]] for productive multiplication. Their nucleic acids have substantially distinct nucleotide sequences from either their helper virus or host. When a satellite subviral agent encodes the coat protein in which it is encapsulated, it is then called a satellite virus. When it does not, it is called a satellite nucleic acid.

Satellite-like nucleic acids resemble satellite nucleic acids, in that they replicate with the aid of helper viruses. However they differ in that they can encode functions that can contribute to the success of their helper viruses; while they are sometimes considered to be genomic elements of their helper viruses, they are not always found within their helper viruses.<ref name="subviral"/>
* Satellite viruses<ref>{{cite journal |last1=Krupovic |first1=Mart |last2=Kuhn |first2=Jens H. |last3=Fischer |first3=Matthias G. |title=A classification system for virophages and satellite viruses |journal=Archives of Virology |date=7 October 2015 |volume=161 |issue=1 |pages=233–247 |doi=10.1007/s00705-015-2622-9|doi-access=free |pmid=26446887 |hdl=11858/00-001M-0000-0028-DC34-F |hdl-access=free }}</ref>
** Single-stranded [[RNA]] satellite viruses
*** Family ''[[Tonesaviridae]]''
**** ''[[Albetovirus]]'' – ''[[Tobacco necrosis satellite virus]]''
**** ''[[Aumaivirus]]'' – ''[[Maize white line mosaic satellite virus]]''
*** Family ''[[Pamosaviridae]]''
**** ''[[Papanivirus]]'' – ''[[Panicum mosaic satellite virus]]''
*** Family ''[[Tomosaviridae]]''
**** ''[[Virtovirus]]'' – ''[[Tobacco mosaic satellite virus]]''
*** Family ''[[Sarthroviridae]]''
**** ''[[Macronovirus]]'' – ''[[Macrobrachium satellite virus 1]]'' (extra small virus)
*** (unnamed genus) – ''[[Nilaparvata lugens commensal X virus]]''
*** (unnamed genus) – ''[[Chronic bee-paralysis satellite virus]]''
** Double-stranded [[DNA]] satellite viruses
*** Family ''[[Lavidaviridae]]'' – Virophages
** Single-stranded [[DNA]] satellite viruses
*** Genus ''[[Dependoparvovirus]]'' – [[Adeno-associated virus]] group
* Satellite nucleic acids
** Single-stranded satellite DNAs
*** Family [[Alphasatellitidae]] (encoding a replication initiator protein)
*** Family [[Tolecusatellitidae]] (encoding a pathogenicity determinant βC1)
** Double-stranded satellite RNAs
** Single-stranded satellite RNAs
*** Subgroup 1: Large linear satellite RNAs
*** Subgroup 2: Small linear satellite RNAs
*** Subgroup 3: Circular satellite RNAs ([[virusoid]]s)
*** Realm ''[[Ribozyviria]]'' / Family ''[[Kolmioviridae]]'' – ''Deltavirus''-like satellite-like RNAs
**** Genus ''[[Deltavirus]]'' – [[Hepadnavirus]]-associated satellite-like RNAs
*** [[Polerovirus]]-associated RNAs
** Satellite-like RNA
** Satellite-like DNA

====Defective interfering particles====
{{main|Defective interfering particle}}

Defective interfering particles are defective viruses that have lost their ability to replicate except in the presence of a helper virus, which is normally the parental virus. They can also interfere with the helper virus.
* Defective interfering particles (RNA)
* Defective interfering particles (DNA)

=== Viriforms ===
Viriforms are a polyphyletic category of [[endogenous viral element]]s. Sometime in their evolution, they became "domesticated" by their host as a key part of the host's lifecycle. The prototypical example is members of the (also polyphyletic) ''[[Polydnaviriformidae]]'', which are used by wasps to send pieces of immunity-blunting DNA into the prey by packing them into [[virus-like particle|virion-like particles]]. Other members are so-called [[gene transfer agent]]s (GTAs) found among prokaryotes. GTA particles resemble [[Caudoviricetes|tailed phages]], but are smaller and carry mostly random pieces of host DNA. GTAs are produced by the host in times of stress; releasing GTAs kills the host cell, but allows pieces of its genetic material to live on in other bacteria, usually of the same species.<ref>{{cite journal |last1=Kuhn |first1=JH |last2=Koonin |first2=EV |title=Viriforms-A New Category of Classifiable Virus-Derived Genetic Elements. |journal=Biomolecules |date=3 February 2023 |volume=13 |issue=2 |page=289 |doi=10.3390/biom13020289 |pmid=36830658 |pmc=9953437 |doi-access=free}}</ref> The three known clades of GTAs, ''[[Rhodogtaviriformidae]]'', ''[[Bartogtaviriformidae]]'', and ''[[Brachygtaviriformidae]]'', all arose independently from different parts of the ''[[Caudoviricetes]]'' family tree.<ref>{{cite journal |last1=Kogay |first1=R |last2=Koppenhöfer |first2=S |last3=Beatty |first3=JT |last4=Kuhn |first4=JH |last5=Lang |first5=AS |last6=Zhaxybayeva |first6=O |title=Formal recognition and classification of gene transfer agents as viriforms. |journal=Virus Evolution |date=2022 |volume=8 |issue=2 |article-number=veac100 |doi=10.1093/ve/veac100 |pmid=36381234 |pmc=9662315}}</ref>

== See also ==
{{Portal|Viruses}}
{{cmn|colwidth=30em|
* [[Glossary of scientific naming]]
* [[Binomial nomenclature]]
* [[Biological classification]]
* [[International Committee on Taxonomy of Viruses]]
* [[List of virus genera]]
* [[List of virus species]]
* [[Nomenclature codes]]
* [[Prion]]
* [[Taxon]]
* [[Taxonomy (biology)|Taxonomy]]
* [[Trinomial nomenclature]]
* [[Virology]]
}}

==Notes==
<references />

== External links ==
{{Wikispecies|Virus}}
* [http://www.ictvonline.org/ ICTV web site] {{Webarchive|url=https://web.archive.org/web/20170203052212/http://www.ictvonline.org/ |date=2017-02-03 }}
* [https://archive.today/20120530101720/http://talk.ictvonline.org/files/ictv_documents/m/msl/1231.aspx ICTV International Committee on Taxonomy of Viruses Master Species List 2009 Version 10 (This is version was published on August 24, 2011)]
* [http://www.virology.net/Big_Virology/BVFamilyGroup.html The Baltimore Method] {{Webarchive|url=https://web.archive.org/web/20000115211904/http://www.virology.net/Big_Virology/BVFamilyGroup.html |date=2000-01-15 }}
* [http://www.viprbrc.org/brc/home.do?decorator=vipr Virus Pathogen Database and Analysis Resource (ViPR)]
* [https://www.youtube.com/watch?v=tQNuThIjGqg How are Viruses Classified?]

{{Baltimore classification}}
{{Taxonomy of...}}
{{Self-replicating organic structures}}

{{DEFAULTSORT:Virus Classification}}
[[Category:Virus taxonomy|classification]]
[[Category:Biological classification]]