Virus Classification

ICTV classification

The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system and wrote guidelines that put greater weight on certain virus properties to maintain family uniformity. A unified taxonomy (a universal system for classifying viruses) has been established. Only a small part of the total diversity of viruses has been studied.
The general taxonomic structure of taxon ranges actually used (as of 2018) is as follows:

Realm (-viria) =>Subrealm (-vira) =>Phylum (-viricota) =>Subphylum (-viricotina) => Class (-viricetes) => Order (-virales) =>Suborder (-virineae) =>Family (-viridae) =>Subfamily (-virinae) => Genus (-virus) => Subgenus (-virus) => Species

As of 2018, 1 realm, 1 phylum, 2 subphyla, 6 classes, 14 orders, 7 suborders, 150 families, 79 subfamilies, 1,019 genera, 59 subgenera, and 5,560 species of viruses have been defined by the ICTV. The realm Riboviria was recognized as the first realm in 2018, and the existing phylum, subphyla, and classes each belong to Riboviria. Riboviria includes all RNA viruses and viroids that replicate by means of RNA-dependent RNA polymerases, which are Baltimore Groups III (dsRNA), IV ((+)ssRNA), and V ((-)ssRNA).

Baltimore classification

The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification.

The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). In addition, ssRNA viruses may be either sense (+) or antisense (−). This classification places viruses into seven groups:

I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
II: ssDNA viruses (+ strand or "sense") DNA (e.g. Parvoviruses)
III: dsRNA viruses (e.g. Reoviruses)
IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g. Picornaviruses, Togaviruses)
V: (−)ssRNA viruses (− strand or antisense) RNA (e.g. Orthomyxoviruses, Rhabdoviruses)
VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate in life-cycle (e.g. Retroviruses)
VII: dsDNA-RT viruses DNA with RNA intermediate in life-cycle (e.g. Hepadnaviruses)

As an example of viral classification, the chickenpox virus, varicella-zoster (VZV), belongs to the order Herpesvirales, family Herpesviridae, subfamily Alphaherpesvirinae, and genus Varicellovirus. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use reverse transcriptase.

The complete set of viruses in an organism or habitat is called the virome; for example, all human viruses constitute the human virome.