Viruses are physical entities that infect living organisms, including plants, animals, and microbes [1]. Viruses may cause disease; further, they can be inoculated or purified, and their genomes can be cloned and sequenced in the laboratory. Viruses have replication cycles that exploit host cell systems, and the word "virus" may refer to the entirety of the cycle or just the extracellular virus particle ("virion" is also used to refer to the virus particle in its extracellular state).

In contrast, virus species are human-made taxonomic categories to which viruses are assigned when they satisfy a particular set of properties, known as "species demarcation criteria" [1]. A virus species is not an entity and cannot be isolated, cloned, sequenced, or make you ill. The notion of a virus species helps us to understand the relationships among viruses, their shared properties, and how they may have evolved.

In a text or presentation, the virus species will usually be mentioned only once, often following, and therefore next to, the virus name when it is introduced for the first time. The virus species is referred to in the context of taxonomy, together with higher taxa, such as the genus and the family in which the virus species is included.

The names of viruses are written differently from the names of virus species

Virus names are often those that are commonly used and known. The format of virus names lies outside the remit of the ICTV; virus names are not standardized as are the names of species. One and the same virus may have several names in the same language or different names in different languages and may be written in different scripts. To ensure that virus names can be clearly distinguishable from species names, advice and illustrative examples are provided below.

Naming viruses

In written communication, virus names should not be italicized, even when they include the name of a host species or genus. In English, all words in a virus name will be written in lowercase letters unless the name includes a proper nounFootnote 1 (e.g., place name) or an alphanumeric designation, which may include uppercase and/or lowercase letters. Virus names may be abbreviated, in which case the abbreviation can be written entirely in uppercase letters or may consist of a combination of uppercase and lowercase letters, with or without numbers. Importantly, how virologists name viruses is not affected by the recent changes in species names [2, 3]. Therefore, the same practices used in the past can continue to be applied for naming new viruses.

Naming virus species

In contrast to the names of viruses, virus species names, like the names of higher taxa, are standardized and assigned through taxonomy proposals that are ratified by the ICTV. A virus species name is always written in italicsFootnote 2 with the first element (the genus name) beginning with an uppercase letter. In the binomial format now mandated by the ICTV [2, 3], the second element, which is the species epithet, is in free form. For example, the epithet may be in the "traditional" binomial format, as used in other taxonomies (usually a Latinized single lowercase word, e.g. Vesiculovirus indiana, see below), or the epithet may include a combination of uppercase and lowercase letters and numbers (e.g. Triavirus phi2958PVL, see below). Binomial species names must never be abbreviated and should never be translated or transliterated.

Examples of correct usage

The symptomless decline of raspberry plants can be caused by raspberry bushy dwarf virus (RBDV), a member of the species Idaeovirus rubi (family Mayoviridae). Idaeovirus rubi is one of two species in the genus Idaeovirus.

Vesicular stomatitis Indiana virus (VSIV) is assigned taxonomically to the family Rhabdoviridae, genus Vesiculovirus, species Vesiculovirus indiana.

The genus Pterovirus includes the species Pterovirus chulinense.

The etiological agents of influenza (influenza virus A, B, C and D) are members of the species Alphainfluenzavirus influenzae, Betainfluenzavirus influenzae, Gammainfluenzavirus influenzae and Deltainfluenzavirus influenzae, respectively.

A new bacteriophage, included in the species Triavirus phi2958PVL, has been isolated.

Aphids transmit potato virus Y (PVY).

Examples of incorrect or misguided usage

Idaeovirus rubi was isolated from an asymptomatic raspberry plant grown in a commercial field in southern Italy. (Incorrect because a species cannot be isolated; the virus name should have been used instead)

Here we describe a new virus from corn for which we propose the name Badnavirus maydis. (Misguided because a virus name should be neither italicized nor binomial. Indeed, this is strongly discouraged by the ICTV)

Results of this study suggest that grapevine virus L (GVL) is a new species in the genus Vitivirus. (Incorrect because a virus cannot be a species; it should read "new virus" or "new member")

The etiological agents of influenza are members of the species Alphainfluenzavirus influenzae, Betainfluenzavirus influenzae, Gammainfluenzavirus influenzae and Deltainfluenzavirus influenzae (family Orthomyxoviridae). (Incorrect because species and family names should be italicized)

A. influenzae is the most common cause of influenza worldwide. (Incorrect because species do not cause disease, and because species names should not be abbreviated)

Virus names in non-English languages

Virus names, just like the names of animals and plants, are used in many languages and should be written following language-specific conventions (Tables 1, 2). For example, in German, all nouns, proper and common, are written with an initial uppercase letter and hence virus names always start with uppercase. In English, only proper nouns are written with an initial uppercase letter. On the other hand, virus species names are unique, are always written in Latin script, and are identical in all languages.

Table 1 Nomenclature for a virus host and host species
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Table 2 Nomenclature for a virus infecting maize and for the virus species
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Useful resources

A complete list of all established virus species can be found at https://ictv.global/taxonomy, and a downloadable Master Species List (MSL) is located at https://ictv.global/MSL. The most recent version of the Virus Metadata Resource (VMR), which provides a list of exemplar (prototype) viruses for each established virus species and links to their genomic sequences, is located at https://ictv.global/vmr.

NOTE: Binomial nomenclature is now the official format for naming virus species in the ICTV virus taxonomy. Although a binomial format has to be used for the naming of new virus species, the current MSL and VMR versions include many species names defined before March 2021 that do not conform to this new format. All virus species names will be gradually changed to binomials over the next three years following ICTV-approved procedures.