The term ‘virus’ is derived from the Latin language meaning poison. Green (1935 A.D.) says, viruses are the smallest units showing the reproductive property considered typical of life. The most accepted definition is the one which was proposed by Bawden in 1943 A.D. He defines virus as “an obligatory parasitic pathogen with at least one dimension of less than two hundred milimicrons.”
Luria (1953 A.D.) defines viruses as submicroscopic entities capable of being introduced into specific living cells and reproducing inside such cells only.
All those viruses that are known till today are parasitic. They are ultramicroscopic. They cannot be seen with a light microscope. Viruses are very minute, infectious agents which are even smaller than the smallest bacteria. They do not have a cellular structure or any metabolic activity and they cannot reproduce outside living cells. Likewise, they are metabolically inactive outside the host cell because they lack enzyme systems and protein synthesis machinery. Viruses can only be recognized by their biological behaviour like by the disease that they cause. Viruses possess certain characteristics of inorganic substances and they also behave like living organisms when they are brought into contact with a healthy plant. They multiply actively like any primitive living thing but are incapable of reproduction in isolated condition.
History of Virus
Adolph Mayer (1885 A.D.), a Dutch scientist and D.J. Ivanowsky (1892 A.D.), a Russian scientist recognized certain microbes as causative agent of mosaic disease of tobacco. The basic criterion used to differentiate these agents from other familiar microbial agents of diseases was their ability to pass through bacteria-proof filters.
W.M. Stanley (1935 A.D.) isolated a virus in a crystalline form and reported that viruses were made exclusively of proteins. Pirie and Bawden (1936 A.D.) established that viruses were nucleoproteins. Harshey and Chase (1952 A.D.) showed that the protein part of virus was noninfectious and that the nucleic acid was responsible for infection.