Lwoff, Home and Touruier (1962) have classified viruses on the basis of their nucleic acids, presence or absence of envelope, symmetry of virus particles, number of protein subunits and diameter of certain viruses. Their system of classification has been approved by the Provisional Committee for the nomenclauture of Viruses (PCNV) in 1965. This system of classification is widely accepted today.
According to Frobischer, 1957 viruses can be grouped into the following four types:
1. Plant Viruses.
2. Mammalian Viruses.
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3. Insect Viruses.
4. Bacterial Viruses or Bacteriophages.
Plant Viruses:
The important diseases are Phyllody of sesamum, Bunchy top of banana, Grassy shoot of sugarcane. Mosaic diseases of beet, bean, cabbage, cucumber, apple, mustard, tomato, tobacco, Spike disease of sandal, Leaf curl of bean, beet, tobacco, cotton, soyabean, zinnia etc.
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Animal viruses:
Common diseases are small pox, hydrophobia, cancer, influenza, polio, rabies, yellow fever, measles, mumps etc.
Bacterial viruses:
The credit of discovering goes to Twort (1915) and D’-Herelle (1917). A bacteriophage consists of a DNA-containing hexagonal head, measuring 950 x 650 A, which is surrounded by a protein sheath. A hollow core, enclosed by a contractile protein sheath extends from the head to a plate at the end of the tail. The bacteriophages invade their host or bacteria by attaching themselves by tail, at first loosely, but then soon irreversibly. The tail then seems to make hole in the cell wall, through which passes the nucleic acid. The empty protein coat is left as a ghost. Though several concepts have been put forward from time to time to explain the nature of bacteriophage, now it is well established that bacteriophages are viruses acting upon bacteria. They are actually widely distributed in nature, i.e., faces of man and animals, sewage water, soil in Ganga water. They are very common, hence, bathing in Holy river cures many bacterial diseases, which may attack human beings.