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Virus classification

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.



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)

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

Group I - dsDNA viruses (double stranded DNA)

Group II - ssDNA viruses (single stranded DNA)

RNA viruses

Group III - dsRNA viruses (double stranded RNA)

Group IV - (+)ssRNA viruses (positive single stranded RNA or mRNA like)

Group V - (-)ssRNA viruses (negative single-stranded RNA)

DNA and RNA Reverse Transcribing viruses

Group VI - ssRNA-RT viruses (single stranded RNA)

Group VII - dsDNA-RT viruses (double stranded DNA)

Subviral agents

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


  • Family Pospiviroidae
    • 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
    • Genus Avsunviroid ; type species: Avocado sunblotch viroid
    • Genus Pelamoviroid ; type species: Peach latent mosaic viroid


  • Satellite viruses
    • Single-stranded RNA satellite viruses
      • Sugroup 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


  • Fungal prions
  • Mamalian prions

Other resources

External links

See also


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