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.
