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Virus Classification

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Virus Classification

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.

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 Influenzaviruses 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


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


Use this resource to answer the questions that follow.

  1. Describe how viruses are classified.
  2. Describe the main feature of the Baltimore classification system.
  3. How are Herpes viruses classified?
  4. How are retroviruses classified?


1. Describe how viruses are classified.

2. What is the difference between viruses in the Herpesvirus family and the Retrovirus family?




Protein coat that surrounds the DNA or RNA of a virus particle.

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