Viruses: Definition, Structure, Classification

  • Susanne ModrowEmail author
  • Dietrich Falke
  • Uwe Truyen
  • Hermann Schätzl
Reference work entry


Viruses are infectious units with diameters of about 16 nm (circoviruses) to over 300 nm (poxviruses; Table 2.1). Their small size makes them ultrafilterable, i.e. they are not retained by bacteria-proof filters. Viruses have evolved over millions of years, and have adapted to specific organisms or their cells. The infectious virus particles, or virions, are composed of proteins and are surrounded in some species of viruses by a lipid membrane, which is referred to as an envelope; the particles contain only one kind of nucleic acid, either DNA or RNA. Viruses do not reproduce by division, such as bacteria, yeasts or other cells, but they replicate in the living cells that they infect. In them, they develop their genomic activity and produce the components from which they are made. They encode neither their own protein synthesis machinery (ribosomes) nor energy-generating metabolic pathways. Therefore, viruses are intracellular parasites. They are able to re-route and modify the course of cellular processes for the optimal execution of their own reproduction. Besides the genetic information encoding their structural components, they additionally possess genes that code for several regulatory active proteins (such as transactivators) and enzymes (e.g. proteases and polymerases).


Prion Disease Bovine Spongiform Encephalopathy Infectious Virus Particle Human Prion Disease Protein Synthesis Machinery 
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Further Reading

  1. Chiu W, Burnett RM, Garcea RL (1997) Structural biology of viruses. Oxford University Press, New YorkGoogle Scholar
  2. Fauquet CM, Mayo MA, Maniloff J, Desselberger U, Ball LA (2005) Virus taxonomy. VIIIth report of the international committee on taxonomy of viruses. Academic, San DiegoGoogle Scholar
  3. Fraenkel-Conrat H (1985) The viruses. Catalogue, characterization, and classification. Plenum, New YorkGoogle Scholar
  4. International Committee on Taxonomy of Viruses (2012) ICTV home.
  5. Knipe DN, Howley PM (eds) (2006) Fields virology, 5th edn. Lippincott-Raven, New YorkGoogle Scholar
  6. Nermuth MV, Steven AC (1987) Animal virus structure. Elsevier, AmsterdamGoogle Scholar
  7. Richman DD, Whitley RJ, Hayden FG (2002) Clinical virology, 2nd edn. ASM Press, Washington, DCGoogle Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2013

Authors and Affiliations

  • Susanne Modrow
    • 1
    Email author
  • Dietrich Falke
    • 2
  • Uwe Truyen
    • 3
  • Hermann Schätzl
    • 4
  1. 1.Inst. Medizinische, Mikrobiologie und HygieneUniversität RegensburgRegensburgGermany
  2. 2.MainzGermany
  3. 3.Veterinärmedizinische Fak., Inst. Tierhygiene undUniversität LeipzigLeipzigGermany
  4. 4.Helmholtz Zentrum München, Institut für VirologieTU MünchenMunichGermany

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