| Virus classification | | | | ssRNA (Group IV) viruses with vertebrate hosts, |
| Virus classification involves naming and placing viruses | | | | Picornavirales contains small (+) strand ssRNA viruses |
| into a taxonomic system. Like the relatively consistent | | | | that infect a variety of plant, insect, and animal hosts, |
| classification systems seen for cellular organisms, virus | | | | and Tymovirales contains monopartite ssRNA viruses |
| classification is the subject of ongoing debate and | | | | that infect plants. Other variations occur between the |
| proposals. This is largely due to the pseudo-living | | | | orders, for example, Nidovirales are isolated for their |
| nature of viruses, which are not yet definitively living or | | | | differentiation in expressing structural and non-structural |
| non-living. As such, they do not fit neatly into the | | | | proteins separately. However, this system of |
| established biological classification system in place for | | | | nomenclature differs from other taxonomic codes on |
| cellular organisms, such as eukaryotes and | | | | several points. A minor point is that names of orders |
| prokaryotes. | | | | and families are italicized, as in the ICBN.[1] Most |
| Virus classification is based mainly on phenotypic | | | | notably, species names generally take the form of |
| characteristics, including morphology, nucleic acid type, | | | | [Disease] virus. The establishment of an order is based |
| mode of replication, host organisms, and the type of | | | | on the inference that the virus families contained within |
| disease they cause. A combination of two main | | | | a single order have most likely evolved from a |
| schemes is currently in widespread use for the | | | | common ancestor. The majority of virus families |
| classification of viruses. David Baltimore, a Nobel | | | | remain unplaced. Currently (2009) 6 orders, 87 families, |
| Prize-winning biologist, devised the Baltimore | | | | 19 subfamilies, 348 genera, and 2,288 species of virus |
| classification system, which places viruses into one of | | | | have been defined[2]. |
| seven groups. These groups are designated by | | | | Baltimore classification |
| Roman numerals and separate viruses based on their | | | | The Baltimore Classification of viruses is based on the |
| mode of replication, and genome type. Accompanying | | | | method of viral mRNA synthesisBaltimore classification |
| this broad method of classification are specific naming | | | | (first defined in 1971) is a classification system that |
| conventions and further classification guidelines set out | | | | places viruses into one of seven groups depending on |
| by the International Committee on Taxonomy of | | | | a combination of their nucleic acid (DNA or RNA), |
| Viruses. | | | | strandedness (single-stranded or double-stranded), |
| ICTV classification | | | | Sense, and method of replication. Other classifications |
| The International Committee on Taxonomy of Viruses | | | | are determined by the disease caused by the virus or |
| began to devise and implement rules for the naming | | | | its morphology, neither of which are satisfactory due |
| and classification of viruses early in the 1990s, an | | | | to different viruses either causing the same disease or |
| effort that continues to the present day. The ICTV is | | | | looking very similar. In addition, viral structures are often |
| the only body charged by the International Union of | | | | difficult to determine under the microscope. Classifying |
| Microbiological Societies (IUMS) with the task of | | | | viruses according to their genome means that those in |
| developing, refining, and maintaining a universal virus | | | | a given category will all behave in a similar fashion, |
| taxonomy. The system shares many features with the | | | | offering some indication of how to proceed with |
| classification system of cellular organisms, such as | | | | further research. Viruses can be placed in one of the |
| taxon structure. Viral classification starts at the level of | | | | seven following groups:[3] |
| order and follows as thus, with the taxon suffixes | | | | • I: dsDNA viruses (e.g. Adenoviruses, |
| given in italics: | | | | Herpesviruses, Poxviruses) |
| Order (-virales) | | | | • II: ssDNA viruses (+)sense DNA (e.g. |
| Family (-viridae) | | | | Parvoviruses) |
| Subfamily (-virinae) | | | | • III: dsRNA viruses (e.g. Reoviruses) |
| Genus (-virus) | | | | • IV: (+)ssRNA viruses (+)sense RNA (e.g. |
| Species | | | | Picornaviruses, Togaviruses) |
| So far, six orders have been established by the ICTV: | | | | • V: (−)ssRNA viruses (−)sense RNA (e.g. |
| the Caudovirales, Herpesvirales, Mononegavirales, | | | | Orthomyxoviruses, Rhabdoviruses) |
| Nidovirales, Picornavirales, and Tymovirales. These | | | | • VI: ssRNA-RT viruses (+)sense RNA with DNA |
| orders span viruses with varying host ranges. | | | | intermediate in life-cycle (e.g. Retroviruses) |
| Caudovirales are tailed dsDNA (group I) | | | | • VII: dsDNA-RT viruses (e.g. Hepadnaviruses) |
| bacteriophages, Herpesvirales contains large | | | | Holmes classification |
| eukaryotic dsDNA viruses, Mononegavirales includes | | | | Holmes (1948) used Carolus Linnaeus's system of |
| non-segmented (-) strand ssRNA (Group V) plant and | | | | binomial nomenclature to classify viruses into 3 groups |
| animal viruses, Nidovirales is composed of (+) strand | | | | under one order, Virales. |