their own genes in maintaining themselves within the biosphere. Finally, valid classifications provide useful guides to our understanding of the origins of various virus groups, and the relationships between viruses in the same group and divergent groups.
Figure 5.2 The structure and relative sizes of a number of (a) DNA and (b) RNA viruses. The largest viruses shown have dimensions approaching 300–400 nm and can be just resolved as refractile points in a high‐quality ultraviolet‐light microscope. The smallest dimensions of viruses shown here are on the order of 25 nm. Classifications of viruses based on the type of nucleic acid serving as the genome and the shape of the capsid are described in the text. ss: Single stranded; ds: double stranded.
The International Committee on Taxonomy of Viruses (ICTV) was created at an international conference on microbiology in Moscow in 1966 in order to develop a single, universal taxonomic scheme for all the viruses infecting animals (vertebrates, invertebrates, and protozoa), plants (higher plants and algae), fungi, bacteria, and archaea. Its membership is made up of distinguished virologists throughout the world, and it has issued periodic reports describing its progress, and its problems, as well as databases containing the properties of viruses and appropriate computer‐based tools for utilizing such databases. One of the notable achievements of this group and the community of virologists that it represents is the recognition of a limited number of viral features that can be used for classification; these include the nature of the viral genome, the presence of an envelope, and the morphology of the virus particle. The classification scheme uses the designation of “family,” even though these phylogenetic terms do not strictly apply in the case of viruses. Table 5.1 lists the families of this scheme, in alphabetical order, as of their 2018 report.
Table 5.1 Classification of viruses according to the ICTV.
| Family | Nature of the genome | Presence of an envelope | Morphology | Genome configuration | Genome size (kbp or kb) | Host |
|---|---|---|---|---|---|---|
| Abyssoviridae | ssRNA | + | ||||
| Ackermannviridae | dsDNA | − | Icosahedral tailed | 1 linear | 155 | Bacteria |
| Adenoviridae | dsDNA | − | Isometric | 1 linear | 28–45 | Vertebrates |
| Alloherpesviridae | dsDNA | + | Isometric | 1 linear | 134–248 | Vertebrates |
| Alphaflexiviridae | ssRNA | − | Filamentous | 1 + segment | 7–9 | Plants |
| Alphasatellitidae | ssDNA | − | N/A | 1 linear | 1.3–1.4 | Plants |
| Alphatetraviridae | ssRNA | − | Isometric | 1 linear | 6.5 | Invertebrates |
| Alvernaviridae | ssRNA | − | Isometric | 1 linear | 4.4 | Dinoflagellates |
| Amalgaviridae | dsRNA | − | Isometric | 1 linear | 3.5 | Plants |
| Ampullaviridae | dsDNA | + | Bottle‐shaped | 1 linear | 23.8 | Archaea |
| Anelloviridae | ssDNA | − | Isometric | 1 linear | 3–4 | Vertebrates |
| Arenaviridae | NssRNA | + | Spherical | 2 ± segments | 11 | Vertebrates |
| Arteriviridae | ssRNA | + | Isometric | 1 + segment | 13–16 | Vertebrates |
| Artoviridae | ssRNA | + | Spherical | 1 − linear | 12.3 | Invertebrates and vertebrates |
| Ascoviridae | dsDNA | + | Reniform | 1 linear | 100–180 | Invertebrates |
| Asfarviridae | dsDNA | + | Spherical | 1 circular | 170–190 | Vertebrates |
| Aspiviridae | ssRNA | − | Filamentous | 3–4 – linear | 11.3–12.5 | Plants |
| Astroviridae | ssRNA |
