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Underlined terms are entries in the glossary - double click to highlight and ctrl+f. A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. From Latin vīrus, which refers to poison and similar liquids. Viruses infect all types of life, from microorganisms to plants and animals. When infected by a virus, a host cell is often forced to rapidly produce thousands of copies of the original virus.Viruses follow the same order of classification as living organisms: life > realm > kingdom > phylum > class > order > family > genus > species.

General Glossary

Term Definition
Morphology The form and structure of organisms as well as their specific structural features. There is external and internal morphologies which respectively refer to features of the outward appearance and features of bones and organs.
Virology The study of biological viruses.
Pathogen An organism or agent that produces disease.
Virus A submicroscopic infectious agent that replicates only inside the living cells of an organism. From Latin vīrus, which refers to poison and similar liquids. Viruses infect all types of life, from microorganisms to plants and animals. When infected by a virus, a host cell is often forced to rapidly produce thousands of copies of the original virus.
  • Viruses are composed of little more than genetic material and certain proteins. This broadly makes them far simpler in structure than most cells.
  • Biologists are generally of the opinion that viruses are not alive due to multiple differences from other entities such as a cellular structure, metabolism, independent reproduction and cellular division to name a few.
  • Viruses follow the same order of classification as living organisms: life > realm > kingdom > phylum > class > order > family > genus > species.
  • The ICTV 2023 virus taxonomy release contained 6 realms, 10 kingdoms, 18 phyla, 2 subphyla, 41 classes, 81 orders, 11 suborders, 314 families, 200 subfamilies, 3522 genera, 84 subgenera and 14690 species.
Virion The independent unit of a virus when it is not inside a cell. They consist of genetic material, a protein coat (capsid) and sometimes an outside envelope of lipids.
  • The shape of virions range from simple helical and icosahedral forms to more complex structures.
Protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues.
Virulence The degree of damage a pathogen inflicts on its host.
Prokaryote A unicellular organism that does not have a nucleus or membrane-bound organelles.
Eukaryote A unicellular or multicellular organism that has a membrane and membrane-bound organelles.
Bacteria Organisms that constitute a large domain of prokaryotic microorganisms.
(Virologic) Realm The highest taxonomic rank for virus established by the ICTV. There are six realms in total and in general they have no genetic relation to each other.
  • Realms are a recent addition to the taxonomic classification of viruses. Prior to 2018, the highest rank was order. Additional ranks have since been added following the introduction of new techniques for comparing highly conserved traits.
Virus Classification Virus classification works from the same premise as classification used for organisms but differs in certain manners.
  • Viruses are classified by phenotype.
  • The usage of the term "kingdom" in virology is not equivalent to its meaning in organism taxonomy where its groupings separate completely different types of organisms.
  • Similarly, "species" refers to a broader collective of viruses compared to how the term is used in other systems. Specifically, it is defined as a group of viruses sharing some but not all traits that constitute a replicating lineage and occupy a particular ecological niche.
  • The full list of ranks and classifying suffixes are as follows:
    • Realm (-viria)
    • Subrealm (-vira)
    • Kingdom (-virae)
    • Subkingdom (-virites)
    • Phylum (-viricota)
    • Subphylum (-viricotina)
    • Class (-viricetes)
    • Subclass (-viricetidae)
    • Order (-virales)
    • Suborder (-virineae)
    • Family (-viridae)
    • Subfamily (-virinae)
    • Genus (-virus)
    • Subgenus (-virus)
    • Species
  • There are several classification ranks that are not ordered within the larger hierarchy because their relation is not yet understood.
International Committee on Taxonomy of Viruses (ICTV) Self-explanatory. Governed by the Virology Division of the International Union of Microbiological Societies (IUMS).
Phenotype The observable characteristics of an organism in contrast to genotype.
Genotype The set of the genetic material of an organism.


Viruses

Virus Traits
Bacteriophage The iconic virus morphology. Bacteriophages infect and replicate within bacteria and archaea. The name comes from "bacteria" and Greek phagein, (to devour).
  • There is an estimated >1031 bacteriophages on the planet, more than every organism combined.