Virus classification

Virus classification involves naming and placing viruses into a taxonomic system. Unlike 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 chracteristics, 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.

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:


 * 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 RNA viruses
 * Group VII: reverse transcribing DNA viruses

ICTV classification
The International Committee on Taxonomy of Viruses devised and implemented several rules on the naming and classification of viruses early in the 1990's. 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 (-virus)

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. 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.

DNA viruses

 * Group I: viruses possess double-stranded DNA and include such virus families as Herpesviridae (herpes viruses), Poxviridae (chickenpox and 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.

RNA viruses

 * Group III: viruses possess double-stranded RNA genomes. 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 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 measles, mumps and rabies.

Reverse transcribing viruses

 * 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.

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

Viroids

 * 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

Satellites

 * Satellite viruses
 * 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