The virus comes from Latin language and means poison. The use of the word goes back to many hundreds of years, long before any one really knew what a virus was, or that it even existed as we know it today. It was generally believed that these 'viruses', or poisons, we're carried in the night air and could cause many unexplained disease. In 1770, a virus disease 'leaf roll' of potato was was observed for the first time in England. In 1785, the same disease was reported from Germany. Virus disease of plants have been described for many years. "Breaking" of tulips, which is now known to be a virus disease was first as early as 1576. Degeneration of potatoes was known in Europe reported in the eighteenth century. Tobacco mosaic virus was recognised by swieten in Holland in 1887. Adolph Mayer a Dutch investigator working with this disease in Holland was first to point out in 1886 that it was readily transmissible and infections Iwanowski a Russian investigator (1892), demonstrated that tobacco mosaic virus would pass through a bacteria proof filter and thereby he distinguished this type of infections agency from bacteria and from fungi. 

Viruses may be divided into three main classes according to the nature of there hosts: plant viruses, animal viruses and bacterial viruses. 

Most of the viruses are crystalline and rod-like, but different in size. Tobacco mosaic viruses is rod-like in shape, 280 micro meter in length and 18 micro meter in breadth. 'Bushy stunt' virus of tomato is 274 micro meter in diameter and its molecular weight is 8,800-12,800,000. Stanley studied the nucleo-proteins of the 'ring spot' virus of tobacco. The virus is rounded with 19 micro meter diameter and 3,400,000 molecular weight. Tobacco  'necrosis' virus is smallest of all the plant viruses known so far. It is rounded with 13-20 micro meter diameter. 

Chemically, viruses consist of nucleoproteins- a combination of nucleic acid and protein-essential constituents of living matter. The more complex viruses contain some fats in addition. Their extreme simplicity leads one to hope that some day it may be possible to produce them in the laboratory if only for the purpose of producing vaccines. 

There are a number of diseases caused by virus infection. Some of these are small-pox, measles, mumps, yellow fever, poliomycilitis (infantile paralysis), usually called polio, chicken pox, rabies, and influenza. It is believed that even common cold is caused by viruses. 

Animal viruses

Some important animal viruses are influenza viruses, Mumps, pox virus, Reo virus etc. In structure Influenza virus differs from TMV in that instead of being straight rigid particles, they are apparently more flexible and form loose coils. A further characteristics is that the coils are enclosed in an envelope, the outside surface of which is covered with minute protein particles probably involved in attaching the virus to the surface of host cells. 

Transmission of viruses

Viruses are transferred from infected plants to healthy ones by a number of agencies described below, and this known as transmission of viruses.

By contact : Viruses can be transmitted merely by contact or slightly rubbing of the infected and healthy plant organs. Such transmission is quite easy in a thickly populated field where one plant is always in close contact with the other and even the slightest movement of the wind can help in rubbing the organs of one with the other. Viruses usually gain entry through the injuries caused on the plant surfaces.