A virus is a non-cellular genetic element that hijacks a cell for its own replication. In its extracellular state the virus particle ,also known as a virion, is metabolically inert and does not carry out respiration or biosynthetic functions. After the infection of a cell, the virus genome is produced and the components that make up the virus particle's protein coat are synthesized from and by the cell's own structural and metabolic components.
The word virus originally meant any poisonous emanation, such as the poison from a snake. Louis Pasteur often referred to pathogenic bacteria as viruses. By the end of the 19th century, a large number of bacteria had been isolated and shown to cause disease, but there were some infectious diseases for which no bacterial cause could be shown. One of these diseases was foot-and-mouth disease, a serious skin disease in animals. In 1898 Freidrich Loeffler and Paul Frosch found the cause. They showed that the agent that caused foot-and-mouth could pass through filters that would stop any bacteria. They showed it was not a toxin by demonstrating that it could cause disease at very low dilutions and was transmitted through filtered material between animals. Over the next few years a number of filterable agents were shown to be the cause of various plant and animal diseases. They came to be known as filterable viruses, but as more work was done on them, the word "filterable" was dropped.
In 1915, the British scientist F.W. Twort discovered a class of virus that infects bacteria; the French scientist F. d'Herelle named them bacteriophages in 1917. Although bacteriophages are still viruses the name has stuck.
The virus genome can be either DNA or RNA based, in either single or double stranded form.
Virus Families
a. DNA Viruses
Double-stranded DNA
Bacteriophages:
Plant Virus Groups:
Animal Virus Groups:
Single-stranded DNA
Bacteriophages:
Plant Virus Groups:
Animal Virus Groups:
b. RNA Viruses
Double-stranded RNA
Bacteriophages
Plant Virus Groups
Animal Virus Groups:
Single-stranded RNA
Bacteriophages
Plant Virus Groups
Animal Virus Groups:
It is now well established that viruses can cause cancer. Certain viruses can bring about a genetic change that results in the initiation of tumour formation. This initiation event could be the activation of a proto-oncogene into an oncogene or the deactivation of a tumour-supressor gene. Once initiation has occurred the cell may remain dormant for some time, until an environmental change brings about promotion. Once a cell has been promoted to the cancerous condition, continued cell-division can result in tumour formation.
Although it is difficult to establish, it is now accepted that the following cancers have viral causes.
Dictionary of Biological Terms, 11th Edition. Lawrence, E (ed). Longman Scientific & Technical, 1995
Biology of Microorganisms, 7th Edition. Brock et al. Prentice Hall International, 1994