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Viruses as aforementioned differ tremendously from other unicellular microorganisms (such as bacteria, fungi, algae and protozoa) in several ways and these shall be highlighted in this section.


  • Viruses reproduce only in living cells. Thus, viruses are obligate intracellular parasites since they only reside and replicate within infected host cells.
  • They lack cellular structure.
  • Viruses generally have the ability to infect other forms of life including bacteria, Archaea, animals, humans and plant cells.
  • Viruses lack functional organelles (for example, ribosomes) for the synthesis of important cellular and metabolic molecules such as proteins. Instead, viruses depend on the cellular and/or metabolic machinery of its infected host cell to synthesize its own proteins and other molecules required for the coupling and release of new virions from the already infected host cell. 
  • Viruses have the ability to integrate their own genome (i.e., DNA or their RNA transcript) into the genome of their host cell.
  • Viruses are (per se) the smallest forms of microorganisms and they usually range from 20 – 300 nm or 350 nm in size. Viruses cannot be seen with the light microscope (whose resolving power is about 300-700 nm). They can only be seen with an electron microscope (with resolving power of about 0.1-10 or 100 nm), whose resolving power is about 1000 times better than the light microscopes. Due to their relatively small sizes; viruses or virions are measured in nanometers (nm). Parvoviruses are among the smallest viruses (about 20 nm in size) while the largest viral family have a size of about 300-350 nm (e.g., smallpox virus). The largest known virus is mimivirus (Figure 1).  
  • Viruses have simple acellular organization that comprises mainly of a particular nucleic acid (DNA or RNA) and a protein coat.
Figure 1. Transmission electron micrograph of mimivirus (also known as giant virus). Mimivirus is the largest known virus; and its shape is as large as that of some small bacteria (for example, E. coli). This virion has a large genome and shape. Mimivirus mainly infect amoeba especially the Acanthamoeba species, which serve as the natural host of mimivirus. However, it is also believed to be causative factor in pneumonia in humans. Photo courtesy: https://www.microbiologyclass.com

Further reading

Acheson N.H (2011). Fundamentals of Molecular Virology. Second edition. John Wiley and Sons Limited, West Sussex, United Kingdom.

Brian W.J Mahy (2001). A Dictionary of Virology. Third edition. Academic Press, California, USA.

Cann A.J (2011). Principles of Molecular Virology. Fifth edition. Academic Press, San Diego, United States.

Carter J and Saunders V (2013). Virology: Principles and Applications. Second edition. Wiley-Blackwell, New Jersey, United States.

Dimmock N (2015). Introduction to Modern Virology. Seventh edition. Wiley-Blackwell, New Jersey, United States.

Kudesia G and Wreghitt T (2009). Clinical and Diagnostic Virology. Cambridge University Press, New York, USA. 

Marty A.M, Jahrling P.B and Geisbert T.W (2006). Viral hemorrhagic fevers. Clin Lab Med, 26(2):345–386.

Strauss J.H and Straus E.G (2008). Viruses and Human Diseases. 2nd edition. Elsevier Academic Press Publications, Oxford, UK.

Zuckerman A.J, Banatvala J.E, Schoub B.D, Grifiths P.D and Mortimer P (2009). Principles and Practice of Clinical Virology. Sixth edition. John Wiley and Sons Ltd Publication, UK.

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