Virus classification is the process of naming viruses and placing them into a taxonomic system. Similar to the classification systems used for cellular organisms, virus classification is the subject of ongoing debate and proposals. This is mainly due to the pseudo-living nature of viruses, which is to say they are non-living particles with some chemical characteristics similar to those of life, or non-cellular life. As such, they do not fit neatly into the established biological classification system in place for cellular organisms.
Viruses are mainly classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. Currently, two main schemes are used for the classification of viruses: the International Committee on Taxonomy of Viruses (ICTV) system and Baltimore classification system, which places viruses into one of seven groups. Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the ICTV.
A catalogue of all the world's viruses has been proposed; some related preliminary efforts have been accomplished.
Species form the basis for any biological classification system. The ICTV had adopted the principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche. 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."
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, this system of nomenclature differs from other taxonomic codes on several points. A minor point is that names of orders and families are italicized, 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 order and continues as follows, with the taxon suffixes given in italics:
Species names often take the form of [Disease] virus, particularly for higher plants and animals.
The establishment of an order is based on the inference that the virus families it contains have most likely evolved from a common ancestor. The majority of virus families remain unplaced. As of 2017, 9 orders, 131 families, 46 subfamilies, 803 genera, and 4,853 species of viruses have been defined by the ICTV. The orders are the Caudovirales, Herpesvirales, Ligamenvirales, Mononegavirales, Nidovirales, Ortervirales, Picornavirales, Bunyavirales and Tymovirales. These orders span viruses with varying host ranges. The Ortervirales (Groups VI and VII), containing also retroviruses (infecting animals including humans e.g. HIV), retrotransposons (infecting invertebrate animals, plants and eukaryotic microorganisms) and caulimoviruses (infecting plants), are the most recent addition to the classification system orders.
Ortervirales contain single-stranded RNA and DNA viruses that replicate through a DNA intermediate (Groups VI and VII).
Tymovirales contain monopartite (+) ssRNA viruses that infect plants.
Bunyavirales contain tripartite (-) ssRNA viruses (Group V).
Other variations occur between the orders: Nidovirales, for example, are isolated for their differentiation in expressing structural and nonstructural proteins separately.
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. 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 existing ICTV classification scheme.
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, 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:
|Virus family||Examples (common names)||Virion
|Nucleic acid type||Group|
|1. Adenoviridae||Adenovirus, infectious canine hepatitis virus||Naked||Icosahedral||ds||I|
|2. Papovaviridae||Papillomavirus, polyomaviridae, simian vacuolating virus||Naked||Icosahedral||ds circular||I|
|3. Parvoviridae||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, cow pox virus, sheep pox virus, orf virus, monkey pox virus, vaccinia virus||Complex coats||Complex||ds||I|
|6. Hepadnaviridae||Hepatitis B virus||Enveloped||Icosahedral||circular, partially ds||VII|
|7. Anelloviridae||Torque teno virus||Naked||Icosahedral||ss circular||II|
|Virus Family||Examples (common names)||Capsid
|Nucleic acid type||Group|
|1. Reoviridae||Reovirus, rotavirus||Naked||Icosahedral||ds||III|
|2. Picornaviridae||Enterovirus, rhinovirus, hepatovirus, cardiovirus, aphthovirus, poliovirus, parechovirus, erbovirus, kobuvirus, teschovirus, coxsackie||Naked||Icosahedral||ss||IV|
|3. Caliciviridae||Norwalk virus||Naked||Icosahedral||ss||IV|
|4. Togaviridae||Rubella virus, alphavirus||Enveloped||Icosahedral||ss||IV|
|5. Arenaviridae||Lymphocytic choriomeningitis virus||Enveloped||Complex||ss(-)||V|
|6. Flaviviridae||Dengue virus, hepatitis C virus, yellow fever virus, Zika virus||Enveloped||Icosahedral||ss||IV|
|7. Orthomyxoviridae||Influenzavirus A, influenzavirus B, influenzavirus C, isavirus, thogotovirus||Enveloped||Helical||ss(-)||V|
|8. Paramyxoviridae||Measles virus, mumps virus, respiratory syncytial virus, Rinderpest virus, canine distemper virus||Enveloped||Helical||ss(-)||V|
|9. Bunyaviridae||California encephalitis virus, hantavirus||Enveloped||Helical||ss(-)||V|
|10. Rhabdoviridae||Rabies virus||Enveloped||Helical||ss(-)||V|
|11. Filoviridae||Ebola virus, Marburg virus||Enveloped||Helical||ss(-)||V|
|12. Coronaviridae||Corona virus||Enveloped||Helical||ss||IV|
|14. Bornaviridae||Borna disease virus||Enveloped||Helical||ss(-)||V|
|15. Arteriviridae||Arterivirus, equine arteritis virus||Enveloped||Icosahedral||ss||IV|
|16. Hepeviridae||Hepatitis E virus||Naked||Icosahedral||ss||IV|
The LHT System of Virus Classification is based on chemical and physical characters like nucleic acid (DNA or RNA), Symmetry (Helical or Icosahedral or Complex), presence of envelope, diameter of capsid, number of capsomers. This classification was approved by the Provisional Committee on Nomenclature of Virus (PNVC) of the International Association of Microbiological Societies (1962). It is as follows:
The following agents are smaller than viruses but have only some of their properties.
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.
Prions, named for their description as "proteinaceous and infectious particles", lack any detectable (as of 2002) nucleic acids or virus-like particles. They resist inactivation procedures that normally affect nucleic acids.