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No consistent system for naming viral isolates has been established by their discoverers. For example, among the vertebrate viruses, some are named for the associated diseases (e.g., poliovirus, rabies virus), for the specific type of disease they cause (e.g., murine leukemia virus), or for the sites in the body that are affected or from which they were first isolated (e.g., rhinovirus and adenovirus). Others are named for the geographic locations from which they were first isolated (e.g., Sendai virus [Sendai, Japan] and Coxsackievirus [Coxsackie, NY]) or for the scientists who first discovered them (e.g., Epstein-Barr virus). In these cases, the virus names are capitalized. Some viruses are even named for the way in which people imagined they were contracted (e.g., influenza, for the “influence” of bad air), how they were first perceived (e.g., the giant mimiviruses [Box 1.10], for the fact that they “mimic” bacteria), or totally by whimsy (e.g., Pandoravirus, after Pandora’s jar [later box] of Greek mythology). Finally, combinations of the above designations are also used (e.g., Rous sarcoma virus).

1 Nature of the nucleic acid in the virus particle (DNA or RNA)

2 Symmetry of the protein shell (capsid)

3 Presence or absence of a lipid membrane (envelope)

4 Dimensions of the virion and capsid

The elucidation of evolutionary relationships by analyses of nucleic acid and protein sequence similarities is now the standard method for assigning viruses to a particular family and ordering members within a family. For example, hepatitis C virus was classified as a member of the family Flaviviridae and MERS was assigned to the Coronaviridae based on their genome sequences. However, as our knowledge of molecular properties of viruses and their reproduction has increased, other relationships have become apparent. Hepadnaviridae, Retroviridae, and some plant viruses are classified as different families on the basis of the nature of their genomes. Nevertheless, they are all related by the fact that reverse transcription is an essential step in their reproductive cycles, and the viral polymerases that perform this task exhibit important similarities in amino acid sequence. Another example is the classification of the giant protozoan Mimiviridae as members of a related group called nucleocytoplasmic large DNA viruses (NCLDVs), which includes the Poxviridae that infect vertebrates (Box 1.10).

The International Committee on Taxonomy of Viruses (ICTV), founded by André Lwoff, authorizes and organizes the classification and establishes nomenclature for all viruses. Freely available as a periodically updated, online resource (https://ictv.global/taxonomy), the 2018 report lists orders, families, genera, and species for all known viruses. In addition, it describes numerous viruses that are not yet classified and probably representatives of new genera and/ or families. The ICTV catalog also includes descriptions of subviral agents (satellites, viroids, and prions) and a list of viruses for which information is still insufficient to make assignments. The pace of discovery of new viruses has been accelerated greatly with the application of metagenomic analyses, direct sequencing of genomes from environmental samples, suggesting that we have barely begun to chart the viral universe.

The ICTV nomenclature has been applied widely in both the scientific and medical literature, and therefore we adopt it in this text. In this nomenclature, the Latinized virus family names are recognized as starting with capital letters and ending with -viridae, as, for example, in the family name Parvo-viridae. These names are used interchangeably with their common derivatives, as, for example, parvoviruses (see additional examples in the Appendix).