Editing Virology (section)

==Virus classification==
{{for|how viruses are classified with relation to other living things|Tree of life (biology)}}
{{Main|Virus classification}}
A major branch of virology is [[virus classification]]. It is artificial in that it is not based on evolutionary [[phylogenetics]] but it is based shared or distinguishing properties of viruses.<ref name="pmid32740831">{{cite journal |vauthors=Hull R, Rima B |title=Virus taxonomy and classification: naming of virus species |journal=Archives of Virology |volume=165 |issue=11 |pages=2733–2736 |date=November 2020 |pmid=32740831 |doi=10.1007/s00705-020-04748-7 |s2cid=220907379 |url=|doi-access=free }}</ref><ref name="pmid25015480">{{cite book |vauthors=Pellett PE, Mitra S, Holland TC |title=Neurovirology |chapter=Basics of virology |series=Handbook of Clinical Neurology |volume=123 |pages=45–66 |date=2014 |pmid=25015480 |pmc=7152233 |doi=10.1016/B978-0-444-53488-0.00002-X |isbn=9780444534880 |chapter-url=}}</ref> It seeks to describe the diversity of viruses by naming and grouping them on the basis of similarities.<ref name="pmid30039318">{{cite journal |vauthors=Simmonds P, Aiewsakun P |title=Virus classification - where do you draw the line? |journal=Archives of Virology |volume=163 |issue=8 |pages=2037–2046 |date=August 2018 |pmid=30039318 |pmc=6096723 |doi=10.1007/s00705-018-3938-z |url=}}</ref> In 1962, [[André Lwoff]], [[Robert Horne (virologist)|Robert Horne]], and Paul Tournier were the first to develop a means of virus classification, based on the [[Linnaean taxonomy|Linnaean]] hierarchical system.<ref name="pmid14467544">{{cite journal | vauthors = Lwoff A, Horne RW, Tournier P | title = [A virus system] | language = fr | journal = Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences | volume = 254 | pages = 4225–27 | date = June 1962 | pmid = 14467544 }}</ref> This system based classification on [[phylum]], [[class (biology)|class]], [[order (biology)|order]], [[family (biology)|family]], [[genus]], and [[species]]. Viruses were grouped according to their shared properties (not those of their hosts) and the type of nucleic acid forming their genomes.<ref name="pmid13931895">{{cite journal | vauthors = Lwoff A, Horne R, Tournier P | title = A system of viruses | journal = Cold Spring Harbor Symposia on Quantitative Biology | volume = 27 | pages = 51–55 | year = 1962 | pmid = 13931895 | doi = 10.1101/sqb.1962.027.001.008 }}</ref> In 1966, the [[International Committee on Taxonomy of Viruses]] (ICTV) was formed. The system proposed by Lwoff, Horne and Tournier was initially not accepted by the ICTV because the small genome size of viruses and their high rate of mutation made it difficult to determine their ancestry beyond order. As such, the [[Baltimore classification]] system has come to be used to supplement the more traditional hierarchy.<ref name="pmid16105179">{{cite journal | vauthors = Fauquet CM, Fargette D | title = International Committee on Taxonomy of Viruses and the 3,142 unassigned species | journal = Virology Journal | volume = 2 | pages = 64 | date = August 2005 | pmid = 16105179 | pmc = 1208960 | doi = 10.1186/1743-422X-2-64 | doi-access = free }}</ref> Starting in 2018, the ICTV began to acknowledge deeper evolutionary relationships between viruses that have been discovered over time and adopted a 15-rank classification system ranging from realm to species.<ref>{{cite journal|author=International Committee on Taxonomy of Viruses Executive Committee|date=May 2020|title=The New Scope of Virus Taxonomy: Partitioning the Virosphere Into 15 Hierarchical Ranks|journal=Nat Microbiol|volume=5|issue=5|pages=668–674|doi=10.1038/s41564-020-0709-x|pmc=7186216|pmid=32341570}}</ref> Additionally, some species within the same genus are grouped into a ''genogroup''.<ref name="pmid34226482">{{cite journal |vauthors=Khan MK, Alam MM |title=Norovirus Gastroenteritis Outbreaks, Genomic Diversity and Evolution: An Overview |journal=Mymensingh Medical Journal|volume=30 |issue=3 |pages=863–873 |date=July 2021 |pmid=34226482 |doi= |url=}}</ref><ref name="pmid22286874">{{cite journal |vauthors=Eberle J, Gürtler L |title=HIV types, groups, subtypes and recombinant forms: errors in replication, selection pressure and quasispecies |journal=Intervirology |volume=55 |issue=2 |pages=79–83 |date=2012 |pmid=22286874 |doi=10.1159/000331993 |s2cid=5642060 |url=|doi-access=free }}</ref>

=== ICTV classification ===
The ICTV developed the current classification system and wrote guidelines that put a greater weight on certain virus properties to maintain family uniformity. A unified taxonomy (a universal system for classifying viruses) has been established. Only a small part of the total diversity of viruses has been studied.<ref>{{cite journal | vauthors = Delwart EL | title = Viral metagenomics | journal = Reviews in Medical Virology | volume = 17 | issue = 2 | pages = 115–31 | year = 2007 | pmid = 17295196 | doi = 10.1002/rmv.532 | pmc = 7169062 }}</ref> As of 2021, 6 realms, 10 kingdoms, 17 phyla, 2 subphyla, 39 classes, 65 orders, 8 suborders, [[List of virus families and subfamilies|233 families, 168 subfamilies]], [[List of virus genera|2,606 genera, 84 subgenera]], and [[List of virus species|10,434 species]] of viruses have been defined by the ICTV.<ref name = ictv2021>{{cite web|url=https://ictv.global/taxonomy|title=Virus Taxonomy: 2021 Release|website=talk.ictvonline.org|publisher=International Committee on Taxonomy of Viruses|access-date=4 April 2022}}</ref>

The general taxonomic structure of taxon ranges and the suffixes used in taxonomic names are shown hereafter. As of 2021, the ranks of subrealm, subkingdom, and subclass are unused, whereas all other ranks are in use.<ref name = ictv2021/>

:[[Realm (virology)|Realm]] (''-viria'')
::Subrealm (''-vira'')
:::[[Kingdom (biology)|Kingdom]] (''-virae'')
::::Subkingdom (''-virites'')
:::::[[Phylum (biology)|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]]

=== Baltimore classification ===
{{Main|Baltimore classification}}
[[File:VirusBaltimoreClassification.svg|thumb|upright=1.5|alt=A diagram showing how the Baltimore Classification is based on a virus's DNA or RNA and method of mRNA synthesis|The Baltimore Classification of viruses is based on the method of viral [[mRNA]] synthesis.]]
The Nobel Prize-winning biologist [[David Baltimore]] devised the [[Baltimore classification]] system.<ref name="pmid34259570">{{cite journal |vauthors=Koonin EV, Krupovic M, Agol VI |title=The Baltimore Classification of Viruses 50 Years Later: How Does It Stand in the Light of Virus Evolution? |journal=Microbiology and Molecular Biology Reviews |volume=85 |issue=3 |pages=e0005321 |date=August 2021 |pmid=34259570 |doi=10.1128/MMBR.00053-21 |pmc=8483701 |s2cid=235821748 |url=https://hal-pasteur.archives-ouvertes.fr/pasteur-03698244/file/Koonin2021MMBR_a.pdf}}</ref>

The Baltimore classification of viruses is based on the mechanism of [[mRNA]] production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use [[reverse transcriptase]] (RT). In addition, ssRNA viruses may be either [[sense (molecular biology)|sense]] (+) or antisense (−). This classification places viruses into seven groups:
{{Baltimore groups}}
{{Portal|Viruses}}