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Viruses are small genetic materials that can hijack cellular machinery for replication. Virologists monitor around 30,000 types of viruses, but there are probably millions. The International Committee on Taxonomy of Viruses developed a unified taxonomy in 1966, and the Baltimore Classification is commonly used in conjunction. Traditional classification is based on the type of organism they infect.
Viruses are tiny bits of genetic material, wrapped in protein shells, capable of hijacking the cellular machinery of plants, animals and bacteria for self-replication. They are extremely small, much smaller than bacteria. A typical virus is between 10 and 300 nanometers (billionths of a meter) in size, while typical bacteria are larger than one micrometer (millionth of a meter).
Only about 30,000 types of viruses are monitored by virologists, although there are probably millions of them. Viral classification is made difficult by a lack of fossil evidence – viruses fossilize poorly – and by controversies over whether or not they are living organisms. Unlike other organisms, new viruses sometimes emerge from the genomes of pre-existing organisms, making it difficult to construct consistent family trees. However, that doesn’t stop virologists from trying.
A unified taxonomy for viruses was only developed in 1966 by the International Committee on Taxonomy of Viruses (ICTV). This classification scheme is inspired by Linnaeus’ classification system for other organisms. It contains five levels: order, family, subfamily, genus and species. The order’s level recognition is extremely recent, with only three named orders. Most of the viral families have not yet been sorted out. Currently, the ICTV only recognizes around 1,550 species.
For example, the varicella virus, Varicella zoster (VZV), belongs to the family Herpesviridae, subfamily Alphaherpesvirinae, genus Varicellovirus, and lacks order-based categorization.
A second classification scheme, the Baltimore Classification, developed and named after Nobel Prize-winning biologist David Baltimore, is commonly used in conjunction with the ICTV system. The Baltimore classification system has classified viruses based on what they are made of. Here is the scheme:
Group ContainsI double-stranded DNA virusII single-stranded DNA virusIIIdouble-stranded RNA virusIV(+)single-stranded RNA virusV(-)single-stranded RNA virusVIdouble stranded reverse transcription virus VIIdouble stranded reverse transcription virus
By using these two classification systems together, virologists can actually get a sense of what they are talking about to each other. Another traditional classification system is based on the type of organism they infect: there are plant-specific viruses, animal-specific viruses, and bacteria-specific viruses, called bacteriophages. Depending on which host the virus has evolved to infect, it will have a different physical structure.
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