Viruses are ultramicroscopic, acellular, infectious, nucleoproteinous, obligate parasites which can cross the bacterial filter.
The extracellular forms of viruses are inert particles and are known as virions. Sometimes the terms ‘viruses’ and ‘virions’ are interchangeably used.
On the basis of the host cell they infect, viruses are called as zoophages (animal viruses) phytophages (plant viruses), phycophages (algal viruses), mycophages (fungal viruses), zymophages (yeast viruses), bacteriophages (bacterial viruses), cyanophages (infecting cyanobacteria), coliphages (infecting E. coli bacteria).
Viruses are visible only under the electron microscope (EM). But interestingly viruses were discovered in the nineteenth century, before the invention of EM by Knoll and Ruska (1934). The discovery of virus cannot be credited to any one scientist; rather it was the contribution of many scientists.
Louis Pastuer (1884):
He coined the term ‘viruses’ (L. virus = poison) to pathogens that are smaller than bacteria. He first demonstrated that viral diseases like rabies can be transmitted for one host to another.
Adolf Mayer (1886):
He first discovered the mosaic disease of tobacco. D.J. Iwanowsky (1892)- He first discovered that the TMV (Tobacco Mosaic Virus) causing mosaic disease of tobacco is smaller than any bacteria and can cross easily through porcelain bacterial filter.
M. Beijerinck (1898):
He further established that viruses like TMV can replicate or proliferate only in growing plant cell.
W.M. Stanley (1935):
He first isolated and crystallized TMV. This finding proves the non-living nature of viruses. For this he shared the 1946 Nobel Prize with Northrop and Sumner who had earlier crystallized the enzyme urease for the first time. Stanley is known as the ‘father of virology’.
Bowden & Pirie (1937):
They first established that viruses are nucleoprotein particles:
i. D.J. Ivanowski (1892) discovered the 1st virus – TMV from Tobacco leaf.
ii. M.W. Beijerink (1898) called the infection fluids as contagium fludium.
iii. F. W. Twort (1915) discovered the bacterial virus.
iv. d Herelle (1916) used the term ‘bacteriophage’ for bacterial viruses.
v. Stanley (1935) isolated and crystallised TMV in pure form.
vi. Hershey and Chase (1952) proved that the nucleic acid is the infectious material and carries the genetic information of the viruses confirming DNAs as the chemical basis of heredity.
vii. Lowff discovered temperate phage and described lysogeny.
viii. Luc Montagnier discovered AIDS virus.
Salient features of viruses:
1. Viruses are acellular and can easily be crystallized.
2. They are ultramicroscopic and filterable (pass through bacteria proof filter).
3. Viruses are obligate intracellular parasites and infect all the major biological groups-animal, plants and bacteria.
4. They are host specific i.e. a specific virus infects a specific host.
5. Each virus particle or virion is nucleoprotein in nature.
6. Nucleic acid is either DNA or RNA but not the both as in any cell.
7. They use enzymes and other machinery of host cell for replication and protein synthesis.
8. They are resistant to chemicals, alcohols and environmental changes.
9. They are easily transmitted from infected host to healthy one through vectors like insects, nematodes etc.
10. Viruses that kill their host are called virulent and those do not always kill their hosts are called temperate or moderate viruses.