Classification of virus 

The taxonomic system used for viruses is artificial (i.e., it does not reflect virus evolution) and is based on the following morphological and biochem­ical criteria:

-Genome: DNA or RNA genome (important basic differentiation of virus types!) as well as configuration of nucleic acid structure: single-stranded (ss) or double-stranded (ds); RNA viruses are further subclassified accord­ing to plus and minus polarity.

-Capsid symmetry: cubic, helical, or complex symmetry.

-Presence or absence of an envelope.

-Diameter of the virion, or of the nucleocapsid with helical symmetry.

The origins and evolution of the viruses are still largely in the dark. In contrast to the taxonomic systems used to classify the higher forms of life, we are therefore unable to classify viruses in such evolutionary systems. An interna­tional nomenclature committee groups viruses according to various criteria and designates these groups, analogously to the higher forms, as families, genera, and species. Despite this element of “artificiality” in the system now in use, the groups appear to make biological sense and to establish order in the enormous variety of known viruses (see Table 1, based on publica­tions by the International Committee on Taxonomy of Viruses).

Table 1 Taxonomy of the Viruses

References

Fritz H. Kayser, M.D. Emeritus Professor of Medical Microbiology Institute of Medical Microbiology, University of Zurich, Zurich, SwitzerlandThieme 2005, Stuttgart ! New York.