Virus Classification


Taxonomic Criteria 

Hosts of Viruses 

Cross-Kingdom Viruses Cross-Phylum Viruses Other Resources 


A universal system for classifying viruses, and a unified taxonomy, has been established by the International Committee on Taxonomy of Viruses (ICTV) since 1966. The system makes use of a series of ranked taxons, with the:

For example, the Ebola virus from Kikwit is classified as:

- Order Mononegavirales

- Family Filoviridae

- Genus Filovirus

- Species: Ebola virus Zaire

The current (2005) Eighth Report of the ICTV lists more than 5,400 viruses in 1938 species, 287 genera, 73 families and 3 orders.

Orders include:

These are presumed to constitute the highest level of "monophyletic" groups of viruses, with a common ancestry, so far recognised.

A comprehensive alphabetical list of viruses appears at the ICTVdb site here.  This includes virus-like agents.

Virus Orthography, or spelling and usage conventions

This format refers only to official taxonomic entities: these are concepts, while viruses are real.

Vernacular or common-use forms of names are neither capitalised nor written in italics.  Thus, while Tobacco mosaic virus refers to the species and Tobamovirus to the taxonomic genus, tobacco mosaic viruses or tobamoviruses are the entities that you work with.

 Taxonomic Criteria

The most important criteria are:

Host Organism(s): eukaryote; prokaryote; vertebrate, etc.

Particle Morphology: filamentous; isometric; naked; enveloped

Genome Type: RNA; DNA; ss- or ds-; circular; linear

- although a number of other criteria - such as

- are important in precise identification, consideration of the above three criteria - and in many cases, just morphology - are sufficient in most cases to allow identification of a virus down to familial if not generic level.

An interactive virus identification key was developed at this site: test it and see how the criteria rapidly allow identification.

Hosts of viruses

These include all classes of cellular organisms described to date:






Eukaryotes: Algae Plants Protozoa Fungi
Invertebrates Vertebrates

This page links to a comprehensive listing of viruses by host infected.

  Cross-Kingdom Viruses

Certain virus families / groupings cross "kingdom" or phylum boundaries.

For example, virus families infecting two kingdoms of organisms are:

The reasons for this are probably caught up in their evolution and cospeciation with their hosts (see also here). 

For example, bunyaviruses and rhabdoviruses probably originated in their present form in insects, and were spread to plants and other animals after insects emerged from the seas and preyed on other terrestrial hosts.

The dsDNA cryptoviruses most probably originated in fungi, and were spread to land plants after these emerged into the terrestrial environment, as fungi began to parasitise them.

The phycodnaviruses - all of which are found in aquatic environments - could well have started out in the progenitors of protozoa and aquatic / marine plants.


Cross-Phylum Viruses

Virus families infecting across different phyla (all infecting insects and vertebrates) are:

These viruses almost certainly spread from insects to vertebrates, after vertebrates joined insects on dry land and insects began to feed on them.  An example here is West Nile flavivirus, which infects mosquitoes and birds, and can be transmitted to humans and other animals as well.

CDC: West Nile Virus - Flowchart: West Nile Virus Transmission Cycle via kwout

Other Resources:

An excellent resource for virus taxonomy is the International Committee on Virus Taxonomy (ICTV) Database, the ICTVdB, maintained by Cornelia Büchen-Osmond and others.

A very good set of pages can be found at the site.

The ICTV virus database (ICTVdb) has a mirror Web site: visit it at the National Center for Biotechnology Information server.


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Copyright Ed Rybicki unless otherwise stated, October 1996; November 1997; August 1998; May 2008