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.

Classification systems

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 ^* 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.