Virus classification

Origin of viruses

Virus classification entails the naming and systematic placement of viruses into a taxonomic framework. Unlike the cellular organisms, such as plants and animals, viruses pose a unique challenge due to their distinct and ambiguous nature which doesn't fit the established biological classification systems, sparking continuous debates and proposals in the scientific community.

Viral Nature and Its Impact on Classification

Viruses, due to their pseudo-living status—neither completely living nor non-living—don't seamlessly align with the classification system used for cellular entities. Their classification draws heavily on phenotypic characteristics, encompassing aspects such as morphology, nucleic acid type, replication mode, the host range, and the specific diseases they instigate.

Present-Day Classification Systems

Two principal classification schemes have gained acceptance in the scientific community:

• Baltimore classification system, developed by the Nobel laureate biologist David Baltimore, segregates viruses based on genome type and replication methods.
• The International Committee on Taxonomy of Viruses (ICTV) offers specific naming conventions and further guidelines, adding depth to the Baltimore system.

Baltimore Classification

The Baltimore classification is predicated on the genome of the virus, namely its nucleic acid type (DNA or RNA), its strandedness (single or double), and replication methodology. Such a genome-based classification provides insights for researchers, as viruses within a category exhibit similar behavior. Under this system, viruses are placed in one of seven groups:

• Group I: Double-stranded DNA viruses. Examples include Herpesviridae, Poxviridae, and certain bacteriophages.
• Group II: Single-stranded DNA viruses, such as Parvoviridae.
• Group III: Double-stranded RNA viruses.
• Group IV: Positive-sense single-stranded RNA viruses, like the SARS virus and yellow fever virus.
• Group V: Negative-sense single-stranded RNA viruses. This group includes viruses like Ebola and measles.
• Group VI: Reverse transcribing RNA viruses, with HIV as a notable member.
• Group VII: Reverse transcribing DNA viruses, including hepatitis B virus.

ICTV Classification

Established in the 1990s, the ICTV's classification system outlines the hierarchical taxonomy of viruses, bearing resemblances to cellular organism classification. However, some differences do exist:

• Names of orders and families are italicized.
• Species names typically adopt the format of [Disease] Virus.

The hierarchical structure is:

• Order (-virales)
• Family (-viridae)
• Subfamily (-virinae)
• Genus (-virus)
• Species (-virus)

To date, only three orders have been named, and numerous families remain unplaced, with around 80 recognized families and 4,000 species of viruses.

Subviral Agents

Smaller entities resembling viruses in some aspects, but not classified as true viruses include:

• Viroids: Examples are Potato spindle tuber viroid and Peach latent mosaic viroid.
• Satellites: They consist of satellite viruses like Chronic bee-paralysis satellite virus, and satellite nucleic acids that can be single-stranded DNAs or RNAs.

See Also

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

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