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Viruses can be classified in several ways, such as by their geometry, by whether they have envelopes, by the identity of the host organism they can infect, by mode of transmission, or by the type of disease they cause. The most useful classification is probably by the type of nucleic acid the virus contains and its mode of expression. This classification was proposed by Nobel-prize winner David Baltimore. It should be noted that none of these classifications are expected to be phylogenetic, as viruses may not share a common origin.
Overview
The various forms of viruses arise because one of the two strands of DNA in which all cellular life forms store their genetic information is redundant, so that viruses can have either single-stranded or double-stranded genomes. Furthermore, some viruses store their genome in RNA rather than in DNA form. RNA arises in cells as an intermediate when genes are translated into proteins. RNA genomes of viruses can be encoded in two different directions: Either the genes are stored in the 5'->3' direction (positive or + polarity), analogous to the direction in which genes are represented in mRNA in cells, or the genes are stored in the opposite direction (negative or - polarity).
The taxonomy of viruses is similar to that of cellular organisms:
- Order (-virales)
- Family (-viridae)
- Subfamily (-virinae)
- Genus (-virus)
- Species
However, the code of nomenclature regulated by the International Committee on Taxonomy of Viruses differs from the others on several points. Most notably, names of orders and families are italicized, and species names are not binomial - instead, they 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.
Classification by genome type
DNA viruses
dsDNA viruses (double stranded DNA) - Order Caudovirales (tailed bacteriophages).
- Family Myoviridae
- Genus T4-like viruses; type species: Enterobacteria phage T4
- Genus P1-like viruses; type species: Enterobacteria phage P1
- Genus P2-like viruses; type species: Enterobacteria phage P2
- Genus Mu-like viruses; type species: Enterobacteria phage Mu
- Genus SPO1-like viruses; type species: Bacillus phage SPO1
- Genus φH-like viruses; type species: Halobacterium virus φH
- Family Podoviridae
- Genus T7-like viruses; type species: Enterobacteria phage T7
- Genus φ29-like viruses; type species: Bacillus phage φ29
- Genus P22-like viruses; type species: Enterobacteria phage P22
- Family Siphoviridae
- Genus λ-like viruses; type species: Enterobacteria phage λ
- Genus T1-like viruses; type species: Enterobacteria phage T1
- Genus T5-like viruses; type species: Enterobacteria phage T5
- Genus c2-like viruses; type species: Lactococcus phage c2
- Genus L5-like viruses; type species: Mycobacterium phage L5
- Genus ψM1-like viruses; type species: Methanobacterium ψM1
- Unassigned families:
- Family Adenoviridae
- Genus Mastadenovirus; type species: Human adenovirus C
- Genus Aviadenovirus; type species: Fowl adenovirus A
- Genus Atadenovirus; type species: Ovine adenovirus D
- Genus Siadenovirus; type species: Turkey adenovirus B
- Family Herpesviridae
- Subfamily Alphaherpesvirinae
- Genus Simplexvirus; type species: Human herpesvirus 1 or Herpes simplex; diseases: cold sores, genital herpes
- Genus Varicellovirus; type species: Human herpesvirus 3 or Varicella-zoster virus; diseases: chickenpox, shingles
- Genus Mardivirus; type species: Gallid herpesvirus 2
- Genus Iltovirus; type species: Gallid herpesvirus 1
- Subfamily Betaherpesvirinae
- Subfamily Gammaherpesvirinae
- Unassigned subfamily:
- Ictalurivirus; type species: Ictalurid herpesvirus 1
- Family Papillomaviridae
- Family Polyomaviridae
- Family Poxviridae
- Subfamily Chordopoxvirinae
- Genus Orthopoxvirus; type species: Vaccinia virus; diseases: cowpox, vaccinia, smallpox
- Genus Parapoxvirus; type species: Orf virus
- Genus Avipoxvirus; type species: Fowlpox virus
- Genus Capripoxvirus; type species: Sheeppox virus
- Genus Leporipoxvirus; type species: Myxoma virus
- Genus Suipoxvirus; type species: Swinepox virus
- Genus Molluscipoxvirus; type species: Molluscum contagiosum virus
- Genus Yatapoxvirus; type species: Yaba monkey tumor virus
- Subfamily Entomopoxvirinae
- Genus Entomopoxvirus A; type species: Melolontha melolontha entomopoxvirus
- Genus Entomopoxvirus B; type species: Amsacta moorei entomopoxvirus
- Genus Entomopoxvirus C; type species: Chironomus luridus entomopoxvirus
ssDNA viruses (single stranded DNA) - Unassigned families of bacteriophages:
- Family Inoviridae
- Genus Inovirus; type species: Enterobacteria phage M13
- Genus Plectrovirus; type species: Acholeplasma phage MV-L51
- Family Microviridae
- Genus Microvirus; type species: Enterobacteria phage φX174
- Genus Spiromicrovirus; type species: Spiroplasma phage 4
- Genus Bdellomicrovirus; type species: Bdellovibrio phage MAC1
- Genus Chlamydiamicrovirus; type species: Chlamydia phage 1
- Unassigned families of viruses:
- Family Geminiviridae
- Genus Mastrevirus; type species: Maize streak virus
- Genus Curtovirus; type species: Beet curly top virus
- Genus Begomovirus; type species: Bean golden mosaic virus
- Family Circoviridae
- Genus Circovirus; type species: Porcine circovirus
- Genus Gyrovirus; type species: Chicken anaemia virus
- Family Nanoviridae
- Genus Nanovirus; type species: Subterranean clover stunt virus
- Genus Babuvirus; type species: Banana bunchy top virus
- Family Parvoviridae (most depend on coinfection with adenoviruses for growth)
- Subfamily Parvovirinae
- Subfamily Densovirinae
- Genus Densovirus; type species: Junonia coenia densovirus
- Genus Iteravirus; type species: Bombyx mori densovirus
- Genus Brevidensovirus; type species: Aedes aegypti densovirus
RNA viruses
dsRNA viruses (double stranded RNA) - Family Cystoviridae
- Genus Cystovirus; type species: Pseudomonas phage ψ6
- Family Reoviridae
- Genus Orthoreovirus; type species: Mammalian orthoreovirus
- Genus Orbivirus; type species: Bluetongue virus
- Genus Rotavirus; type species: Rotavirus A (causes diarrhea)
- Genus Coltivirus; type species: Colorado tick fever virus
- Genus Aquareovirus; type species: Aquareovirus A
- Genus Cypovirus; type species: Cypovirus 1
- Genus Fijivirus; type species: Fiji disease virus
- Genus Phytoreovirus; type species: Rice dwarf virus
- Genus Oryzavirus; type species: Rice ragged stunt virus
- Family Birnaviridae
- Genus Aquabirnavirus; type species: Infectious pancreatic necrosis virus
- Genus Avibirnavirus; type species: Infectious bursal disease virus
- Genus Entomobirnavirus; type species: Drosophila X virus
- Family Totiviridae
- Genus Totivirus; type species: Saccharomyces cerevisiae virus L-A
- Genus Giardiavirus; type species: Giardia lamblia virus
- Genus Leishmaniavirus; type species: Leishmania RNA virus 1-1
(+)ssRNA viruses (positive single stranded RNA or mRNA like) - Order Nidovirales
- Family Arteriviridae
- Genus Arterivirus; type species: Equine arteritis virus
- Family Coronaviridae
- Family Roniviridae
- Genus Okavirus; type species: Gill-associated virus
- Unassigned families:
- Family Astroviridae
- Genus Mamastrovirus; type species: Human astrovirus
- Genus Avastrovirus; type species: Turkey astrovirus
- Family Barnaviridae
- Genus Barnavirus; type species: Mushroom bacilliform virus
- Family Bromoviridae
- Genus Alfamovirus; type species: Alfalfa mosaic virus
- Genus Ilarvirus; type species: Tobacco streak virus
- Genus Bromovirus; type species: Brome mosaic virus
- Genus Cucumovirus; type species: Cucumber mosaic virus
- Genus Oleavirus; type species: Olive latent virus 2
- Family Caliciviridae
- Genus Vesivirus; type species: Swine vesicular exanthema virus
- Genus Lagovirus; type species: Rabbit hemorrhagic disease virus
- Genus Norovirus; type species: Norwalk virus
- Genus Sapovirus; type species: Sapporo virus
- Family Closteroviridae
- Genus Closterovirus; type species: Beet yellows virus
- Genus Crinivirus; type species: Lettuce infectious yellows virus
- Genus Ampelovirus; type species: Grapevine leafroll-associated virus 3
- Family Comoviridae
- Genus Comovirus; type species: Cowpea mosaic virus
- Genus Fabavirus; type species: Broad bean wilt virus 1
- Genus Nepovirus; type species: Tobacco ringspot virus
- Family Dicistroviridae
- Genus Cripavirus; type species: Cricket paralysis virus
- Genus Iflavirus; type species: Infectious flacherie virus
- Family Flaviviridae
- Genus Flavivirus; type species: Yellow fever virus
- Genus Hepacivirus; type species: Hepatitis C virus
- Genus Pestivirus; type species: Bovine viral diarrhea virus
- Family Flexiviridae
- Genus Allexivirus; type species: Shallot virus X
- Genus Carlavirus; type species: Carnation latent virus
- Genus Capillovirus; type species: Apple stem grooving virus
- Genus Foveavirus; type species: Apple stem pitting virus
- Genus Potexvirus; type species: Potato virus X
- Genus Trichovirus; type species: Apple chlorotic leaf spot virus
- Genus Vitivirus; type species: Grap vine virus A
- Family Hepeviridae
- Genus Hepevirus;type species: Hepatitis E virus
- Family Leviviridae
- Genus Levivirus; type species: Enterobacteria phage MS2
- Genus Allolevivirus; type species: Enterobacteria phage Qβ
- Family Luteoviridae
- Genus Luteovirus; type species: Barley yellow dwarf virus-PAV
- Genus Polerovirus; type species: Potato leafroll virus
- Genus Enamovirus; type species: Pea enation mosaic virus-1
- Family Nodaviridae
- Genus Alphanodavirus; type species: Nodamura virus
- Genus Betanodavirus; type species: Striped jack nervous necrosis virus
- Family Picornaviridae
- Family Potyviridae
- Genus Potyvirus; type species: Potato virus Y
- Genus Rymovirus; type species: Ryegrass mosaic virus
- Genus Bymovirus; type species: Barley yellow mosaic virus
- Genus Macluravirus; type species: Maclura mosaic virus
- Genus Ipomovirus; type species: Sweet potato mild mottle virus
- Genus Tritimovirus; type species: Wheat streak mosaic virus
- Family Sequiviridae
- Genus Sequivirus; type species: Parsnip yellow fleck virus
- Genus Waikavirus; type species: Rice tungro spherical virus
- Family Tetraviridae
- Family Togaviridae
- Genus Alphavirus; type species: Sindbis virus
- Genus Rubivirus; type species: Rubella virus
- Family Tombusviridae
- Genus Tombusvirus; type species: Tomato bushy stunt virus
- Genus Carmovirus; type species: Carnation mottle virus
- Genus Necrovirus; type species: Tobacco necrosis virus A
- Genus Dianthovirus; type species: Carnation ringspot virus
- Genus Machlomovirus; type species: Maize chlorotic mottle virus
- Genus Avenavirus; type species: Oat chlorotic stunt virus
- Genus Aureusvirus; type species: Pothos latent virus
- Genus Panicovirus; type species: Panicum mosaic virus
- Family Tymoviridae
- Genus Tymovirus; type species: Turnip yellow mosaic virus
- Genus Marafivirus; type species: Maize rayado fino virus
- Genus Maculavirus ; type species: Grapevine fleck virus
- Unassigned genus:
- Genus Benyvirus; type species: Beet necrotic yellow vein virus
- Genus Furovirus ; type species: Soil-borne wheat mosaic virus
- Genus Hordeivirus; type species: Barley stripe mosaic virus
- Genus Idaeovirus; type species: Raspberry bushy dwarf virus
- Genus Ourmiavirus; type species: Ourmia melon virus
- Genus Pecluvirus; type species: Peanut clump virus
- Genus Pomovirus; type species: Potato mop-top virus
- Genus Sobemovirus; type species: Southern bean mosaic virus
- Genus Tobamovirus; type species: Tobacco mosaic virus
- Genus Tobravirus; type species: Tobacco rattle virus
- Genus Umbravirus; type species: Carrot mottle virus
(-)ssRNA viruses (negative single-stranded RNA) - Order Mononegavirales (non-segmented negative stranded viruses)
- Segmented negative stranded viruses
- Family Arenaviridae
- Genus Arenavirus; type species: Lymphocytic choriomeningitis virus
- Family Bunyaviridae
- Family Orthomyxoviridae (Influenza viruses)
- Genus Influenzavirus A; type species: Influenza A virus
- Genus Influenzavirus B; type species: Influenza B virus
- Genus Influenzavirus C; type species: Influenza C virus
- Genus Isavirus; type species: Infectious salmon anemia virus
- Genus Thogotovirus; type species: Thogoto virus
- Unassigned genera:
- Genus Deltavirus; type species: Hepatitis delta virus
- Genus Ophiovirus; type species: Citrus psorosis virus
- Genus Tenuivirus; type species: Rice stripe virus
Naked RNA viruses - Family Narnaviridae
- Genus Narnavirus; type species: Saccharomyces cerevisiae 20SRNA narnavirus
- Genus Mitovirus; type species: Cryphonectria parasitica mitovirus-1 NB631
DNA and RNA Reverse Transcribing viruses
ssRNA-RT viruses (single stranded RNA) - Family Metaviridae
- Genus Metavirus; type species: Saccharomyces cerevisiae Ty3 virus
- Genus Errantivirus; type species: Drosophila melanogaster gypsy virus
- Family Pseudoviridae
- Genus Pseudovirus; type species: Saccharomyces cerevisiae Ty1 virus
- Genus Hemivirus; type species: Drosophila melanogaster copia virus
- Family Retroviridae - Retrovirus (leukemia, HIV)
- Genus Alpharetrovirus; type species: Avian leucosis virus
- Genus Betaretrovirus; type species: Mouse mammary tumour virus
- Genus Gammaretrovirus; type species: Murine leukeamia virus
- Genus Deltaretrovirus; type species: Bovine leukaemia virus
- Genus Epsilonretrovirus; type species: Walleye dermal sarcoma virus
- Genus Lentivirus; type species: Human immunodeficiency virus 1
- Genus Spumavirus; type species: Chimpanzee foamy virus
dsDNA-RT viruses (double stranded DNA) - Family Hepadnaviridae
- Genus Orthohepadnavirus; type species: Hepatitis B virus
- Genus Avihepadnavirus; type species: Duck hepatitis B virus
- Family Caulimoviridae
- Genus Badnavirus; type species: Commelina yellow mottle virus
- Genus Caulimovirus; type species: Cauliflower mosaic virus
- Genus Tungrovirus; type species: Rice tungro bacilliform virus
- Genus Soymovirus ; type species: Soybean chlorotic mottle virus
- Genus Cavemovirus; type species: Cassava vein mosaic virus
- Genus Petuvirus; type species: Petunia vein clearing virus
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