7.2: Virus Classification

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How are viruses classified?

In part by their shape. This picture represents a bacteriophage, a virus that infects bacteria. Notice the distinctive shape. This virus has a complex shape.

Classification of Viruses

Like the classification systems for cellular organisms, virus classification is the subject of ongoing debate. This is largely due to the nature of viruses, which are not living organisms by the classic definition, but neither are they necessarily non-living. Therefore, viruses do not fit neatly into the biological classification system of cellular organisms, as plants and animals do.

Virus classification is based mainly on characteristics of the viral particles, including the capsid shape, the type of nucleic acid (DNA or RNA, double stranded (ds) or single stranded (ss)) within the capsid, the process of replication, their host organisms, or the type of disease they cause. The Table below lists characteristics such as capsid shape, presence of an envelope, and the diseases the viruses can cause.

Viruses
Virus Family	Virus	Envelope	Capsid shape	Nucleic Acid	Disease
Adenoviruses	Adenovirus	No	Icosahedral	dsDNA	upper respiratory infections
Parvoviruses	Parvovirus	No	Icosahedral	ssDNA	fifth disease, Canine parvovirus
Herpesviruses	Herpes simplex virus, Varicella zoster virus, Epstein Barr virus	Yes	Icosahedral	dsDNA	Herpes, chicken pox, shingles, infectious mononucleosis
Hepadnaviruses	Hepatitis B virus	Yes	Icosahedral	dsDNA	Hepatitis B
Reoviruses	Rotavirus	No	Icosahedral	dsRNA	gastroenteritis
Retroviruses	HIV, HTLV-I	Yes	Complex	ssRNA	HIV/AIDS, leukemia
Orthomyxoviruses	Influenza viruses	Yes	Helical	ssRNA	Influenza (flu)
Rhabdoviruses	Rabies virus	Yes	Helical	ssRNA	Rabies
Coronaviruses	Corona virus	Yes	Complex	ssRNA	Common cold, Severe acute respiratory syndrome (SARS)
Cystoviruses	Cystovirus	Yes	Icosahedral	dsRNA	Infects Pseudomonas bacteria

Summary

Viruses can be classified on the basis of capsid shape, presence or absence of an envelope, and type of nucleic acid.

Review

Describe how viruses are classified.
What is the difference between viruses in the Herpesvirus family and the Retrovirus family?
What are the four types of nucleic acid that may be present in a virus?
Give an example of a disease caused by a member of the Orthomyxoviruses.
Give an example of a disease caused by a retrovirus.

Image	Reference	Attributions
	[Figure 1]	License: CC BY-NC