VIRUS

Virus is a sub-microscopic, parasitic micro-organism comprising a protein or a protein-lipid sheath containing nucleic acid. Viruses are inert outside living cells, but within appropriate cells they can replicate and give rise to the manifestations of the associated viral disease in the host organism. Various viruses infect animals, plants and bacteria; the viruses infecting bacteria are called bacteriophages. Few drugs act specifically against viruses, although immunity can be induced in susceptible cells against particular viruses. Various pathogenic organisms formerly regarded as large viruses are now distinguished as bedsoniu.
Viruses can be resolved only with electron microscope. Since viruses pass through filters that retain bacteria, they are often called filterable viruses. Tobacco mosaic was the first virus to be crystallized and isolated. It contains some 2000 proteins in a sequence of 158 amino acids. Bushy stunt virus, found in tomato plants has a molecular weight of 7.6 million.
Viruses differ from organisms, in that they are only half alive. They lack metabolism, are unable to utilize oxygen to synthesize macromolecules, to grow or to die. They account for many diseases in plants including, mosaic, ringspots, bunchy top, etc.