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Virus classification involves naming and placing viruses into a taxonomic system. Like the relatively consistent classification systems seen for cellular organisms, virus classification is the subject of ongoing debate and proposals. This is largely due to the pseudo-living nature of viruses, which are not yet definitively living or non-living. As such, they do not fit neatly into the established biological classification system in place for cellular organisms, such as plants and animals, for several reasons.

Virus classification is based mainly on phenotypic characteristics, including morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. A combination of two main schemes is currently in widespread use for the classification of viruses. David Baltimore, a Nobel Prize-winning biologist, devised the Baltimore classification system, which places viruses into one of seven groups. These groups are designated by Roman numerals and separate viruses based on their mode of replication, and genome type. Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the International Committee on Taxonomy of Viruses.

Contents 1 Classification systems 1.1 Baltimore classification 1.2 Holmes classification 1.3 LHT System of Virus Classification 1.4 Casjens and Kings classification of virus 1.5 ICTV classification 2 Virus classification 2.1 DNA viruses 2.2 RNA viruses 2.3 Reverse transcribing viruses 3 Subviral agents 3.1 Viroids 3.2 Satellites 3.3 Prions 4 Notes 5 See also 6 External links

Main article: Baltimore classification

Baltimore classification is a classification system which places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), and method of replication. 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:^[1]

• Group I: double-stranded DNA viruses
• Group II: single-stranded DNA viruses
• Group III: double-stranded RNA viruses
• Group IV: positive-sense single-stranded RNA viruses
• Group V: negative-sense single-stranded RNA viruses
• Group VI: reverse transcribing Diploid single-stranded RNA viruses
• Group VII: reverse transcribing Circular double-stranded DNA viruses

Holmes(1948) used Carolus Linnaeus system of binomial nomenclature classification system to viruses into 3 groups under one order ,order virales.they are placed as follows:

• Group I: phaginae(attacks bacteria)
• Group II: phytophaginae(attacks plants)
• Group III: zoophaginae(attacks animals)

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.^[2] This classification was approved by the Provisional Committee on Nomenclature of Virus (PNVC) of the International Association of Microbiological Societies (1962).^{[citations needed]} It is as follows:

• Phylum Vira(divided into 2 subphyla)

• Subphylum Deoxyvira(DNA viruses)

• Class Deoxybinala(dual symmetry)

• Order Uroviridae

• Family Phagoviridae

• Class Deoxyhelica(Helical symmetry)

• Order Chitovirales

• Family Poxviridae

• Class Deoxycubica(cubical symmetry)

• Order Peplovirales

• Family Herpesviridae(162 capsomeres)

• Order Haplovirales(no envelope)

• Family Iridoviridae(812 capsomeres)
• Family Adenoviridae(252 capsomeres)
• Family Papiloviridae(72 capsomeres)
• Family Paroviridae(32 capsomeres)
• Family Microviridae(12 capsomeres)

• Subphylum Ribovira(RNA viruses)

• Class Ribocubica

• Order Togovirales

• Family Arboviridae

• Order Lymovirales

• Family Napoviridae
• Family Reoviridae

• Class Ribohelica

• Order Sagovirales

• Family Stomataviridae
• Family Paramyxoviridae
• Family Myxoviridae

• Order Rbadovirales

• Suborder Felxiviridales

• Family Mesoviridae
• Family Peptoviridae

• Suborder Rigidovirales

• Family Pachyviridae
• Family Protoviridae
• Family Polichoviridae

Casjens and Kings(1975) classified virus into 4 groups based on type of nucleic acid ,presence of envelope,symmetry and site of assembly^{[citation needed]}. It is as follows:

• Single Stranded RNA Viruses
• Double Stranded RNA Viruses
• Single Stranded DNA Viruses
• Double Stranded DNA Viruses

The International Committee on Taxonomy of Viruses devised and implemented several rules on the naming and classification of viruses early in the 1990s. To this day they oversee the naming and placement of viral species into the framework. The system shares many features with the classification system of cellular organisms, such as taxon structure. Viral classification starts at the level of order and follows as thus, with the taxon suffixes given in italics:

Order (-virales)

Family (-viridae)

Subfamily (-virinae)

Genus (-virus)

Species

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, as in the ICBN.^[3] Most notably, species names generally take the form of [Disease] virus. The recognition of orders is very recent and has been deliberately slow; to date, only three have been named, and most families remain unplaced. Approximately 80 families and 4000 species of virus are known.

For more details on this topic, see DNA virus.

• Group I: viruses possess double-stranded DNA and include such virus families as Herpesviridae (examples like HSV1 (oral herpes), HSV2 (genital herpes), VZV (chickenpox), EBV (Epstein-Barr virus), CMV (Cytomegalovirus)), Poxviridae (smallpox) and many tailed bacteriophages. The mimivirus was also placed into this group.
• Group II: viruses possess single-stranded DNA and include such virus families as Parvoviridae and the important bacteriophage M13.

Virus Family	Virus Genus	Virion- naked/ enveloped	Capsid Symmetry	Type of nucleic acid
1.Adenoviridae	Adenovirus	Naked	Icosahedral	ds
2.Papovaviridae	Papillomavirus	Naked	Icosahedral	ds circular
3.Parvoviridae	B 19 virus	Naked	Icosahedral	ss
4.Herpesviridae	Herpes Simplex Virus, Varicella zoster virus, Cytomegalovirus, Epstein Barr virus	Enveloped	Icosahedral	ds
5.Poxviridae	Small pox virus, Vaccinia virus	Complex coats	Complex	ds
6.Hepadnaviridae	Hepatitis B virus	Enveloped	Icosahedral	ds circular
7.Polyomaviridae	Polyoma virus (progressive multifocal leucoencephalopathy)	?	?	ds

For more details on this topic, see RNA virus.

• Group III: viruses possess double-stranded RNA genomes, e.g. rotavirus. These genomes are always segmented.
• 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), SARS virus, hepatitis C virus, yellow fever virus, and rubella virus.
• Group V: viruses possess negative-sense single-stranded RNA genomes. The deadly Ebola and Marburg viruses are well known members of this group, along with influenza virus, measles, mumps and rabies.

Virus Family	Virus Genera	Virion- naked/ enveloped	Capsid Symmetry	Type of nucleic acid
1.Reoviridae	Reovirus, Rotavirus	Naked	Icosahedral	ds
2.Picornaviridae	Enterovirus, Rhinovirus, Hepatovirus, Cardiovirus, Aphthovirus, Parechovirus, Erbovirus, Kobuvirus, Teschovirus	Naked	Icosahedral	ss
3.Caliciviridae	Norwalk virus, Hepatitis E virus	Naked	Icosahedral	ss
4.Togaviridae	Rubella virus	Enveloped	Icosahedral	ss
5.Arenaviridae	Lymphocytic choriomeningitis virus	Enveloped	Complex	ss
6.Retroviridae	HIV-1, HIV-2, HTLV-I	Enveloped	Complex	ss
7.Flaviviridae	Dengue virus, Hepatitis C virus, Yellow fever virus	Enveloped	Complex	ss
8.Orthomyxoviridae	Influenzavirus A, Influenzavirus B, Influenzavirus C, Isavirus, Thogotovirus	Enveloped	Helical	ss
9.Paramyxoviridae	Measles virus, Mumps virus, Respiratory syncytial virus	Enveloped	Helical	ss
10.Bunyaviridae	California encephalitis virus, Hantavirus	Enveloped	Helical	ss
11.Rhabdoviridae	Rabies virus	Enveloped	Helical	ss
12.Filoviridae	Ebola virus, Marburg virus	Enveloped	Helical	ss
13.Coronaviridae	Corona virus	Enveloped	Complex	ss
14.Astroviridae	Astrovirus	Naked	Icosahedral	ss
15.Bornaviridae	Borna disease virus	Enveloped	Helical	ss

For more details on this topic, see Reverse transcribing virus.

• Group VI: viruses possess single-stranded RNA genomes and replicate using reverse transcriptase. The retroviruses 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.

The following agents are smaller than viruses but have some of their properties.

• Family Pospiviroidae^[4]
  • 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
• Family Avsunviroidae^[5]
  • Genus Avsunviroid; type species: Avocado sunblotch viroid
  • Genus Pelamoviroid; type species: Peach latent mosaic viroid

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's then called a satellite virus.

• Satellite viruses^[6]
  • Single-stranded RNA satellite viruses
    • Subgroup 1: Chronic bee-paralysis satellite virus
    • Subgroup 2: Tobacco necrosis satellite virus
• Satellite nucleic acids
  • Single-stranded satellite DNAs
  • Double-stranded satellite RNAs
  • Single-stranded satellite RNAs
    • Subgroup 1: Large satellite RNAs
    • Subgroup 2: Small linear satellite RNAs
    • Subgroup 3: Circular satellite RNAs (virusoids)

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 which normally affect nucleic acids.^[7]

• Mammalian prions:
  • Agents of spongiform encephalopathies

• Fungal prions:
  • PSI+ prion of Saccharomyces cerevisiae
  • URE3 prion of Saccharomyces cerevisiae
  • RNQ/PIN+ prion of Saccharomyces cerevisiae
  • het-s prion of Podospora anserina

1. ^ "Virus Taxonomy Portal." (Website.) Viral Bioinformatics Resource Center & Viral Bioinformatics - Canada. Retrieved on 2007-09-27.
2. ^ Lwoff A, Horne R, Tournier P (1962). "A system of viruses". Cold Spring Harb. Symp. Quant. Biol. 27: 51–5. PMID 13931895. Retrieved on 2007-10-17. 
3. ^ The Virus Species Concept: Introduction Virus Taxonomy Online: Seventh Report of the International Committee on Taxonomy of Viruses. 2000. Retrieved on 2007-07-14.
4. ^ "80.001 Popsiviroidae - ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2007-09-27.
5. ^ "80.002 Avsunviroidae - ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2007-09-27.
6. ^ "81. Satellites - ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2007-09-27.
7. ^ "90. Prions - ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2007-09-27.

• Binomial nomenclature
• International Committee on Taxonomy of Viruses
• List of viruses
• Scientific classification
• Taxonomy
• Trinomial nomenclature
• Virology
• List of genera of viruses

• ICTV Index of Viruses
• MicrobiologyBytes: 8th ICTV Index classification
• The Baltimore Method