MicrobiologyBytes: Infection & Immunity: Viruses Updated: January 7, 2007 Search


Aims and Objectives

After reading this document, you should be able to:

  1. Discuss what constitutes a living organism.
  2. Define what constitutes a virus.
  3. Have a knowledge of the principles of structure, classification and replication of major viruses of medical importance.
  4. Understand the principles of diagnosis of virus infections.
  5. Know the mechanisms of action of antiviral drugs used in clinical practice.

General Characteristics

Viruses have the following properties:

  1. Obligate intracellular parasites
  2. Small- filterable through bacteriological filters
  3. Contain a single type of nucleic acid, either DNA or RNA
  4. Contain a protein coat (the capsid) consisting of individual protein units (capsomeres)
  5. May contain a host-derived lipid membrane (the envelope) through which may be inserted viral proteins (spikes or peplomers)
  6. Multiply inside living cells by using the biosynthetic machinery of the host

Virus Morphology

Viruses can be classified into several morphological types:

Taxonomy of Viruses

This is on the basis of nucleic acid type and morphology. In future, it will be more likely to be based on nucleic acid sequence.

Major Virus Families That Infect Humans

Family Nucleic Acid Envelope Capsid Example
Adenoviridae dsDNA No Cubic Adenovirus
Arenaviridae ssRNA Yes Complex Lassa fever virus
Bunyaviridae ssRNA Yes Helical Hantaan
Caliciviridae ssRNA No Cubic Norwalk virus
Coronaviridae ssRNA Yes Helical 229E
Filoviridae ssRNA Yes Helical Marburg
Flaviviridae ssRNA Yes Cubic Yellow fever
Hepadnaviridae dsDNA No Cubic Hepatitis B
Herpesviridae dsDNA Yes Cubic Cytomegalovirus
Orthomyxoviridae ssRNA Yes Helical Influenza
Papovaviridae dsDNA No Cubic Papillomavirus
Paramyxoviridae ssRNA Yes Helical Respiratory syncytial
Parvoviridae ssDNA No Cubic B19
Picornaviridae ssRNA No Cubic Rhinovirus
Poxviridae dsDNA Yes Complex Molluscum contagiosum
Reoviridae dsRNA No Cubic Rotavirus
Retroviridae ssRNA Yes Complex Human immunodeficiency virus
Rhabdoviridae ssRNA Yes Helical Rabies
Togaviridae ssRNA Yes Cubic Rubella

Replication of Viruses

There are 5 major steps in the replication cycle of all viruses:

  1. Attachment
  2. Penetration
  3. Nucleic acid and protein synthesis
  4. Assembly of virions
  5. Release/egress

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Virus-Like Agents

Prions - proteinaceous infectious particles, e.g. CJD agent

Viroids - RNA only, e.g. hepatitis D

Clinical Features of Viruses

These are protean, and range from the common cold to AIDS and cancer. See course textbook.

Diagnosis of Viral Infections

This can be by:

Management of Viral Infections

Management of more trivial infections depends on control of symptoms, as there are few antiviral agents available. Principal drugs are:

© AJC 2007.