About a century ago at the time of Louis Pasteur (1822-1895) and Robert Koch (1843-1910), the word “virus” was  generally referred to as a poison associated with disease and death. The present notion of virus is entirely different. Now viruses are recognized as particles of nucleic acid often with a protein coat. They replicate in living cells and cause many diseases such as influenza, hepatitis, small pox and AIDS. In this section the focus is on the properties of viruses and life cycle of bacterial viruses, also known as bacteriophages. Some diseases caused by viruses shall also be discussed in this section. The branch which deals with the study of viruses is known as virology.

The word virus is derived from Latin word venome meaning poisonous fluid. It can be defined as non cellular infectious entities which contain either RNA or DNA, normally encased in proteinaceous coat, and reproduce only in living cells. Viruses utilize the biosynthetic machinery of the host for its own synthesis and then transfer themselves efficiently to other Cell.

Viruses

Some viral diseases have been known for centuries. In fact, the first infectious disease against which effective method of prevention was developed was a viral disease. In 1796, Edward Jenner first vaccinated an 8 years old boy with material removed from cowpox lesion on the hand of milkmaid. After six weeks the boy was inoculated with pus from a small pox victim, but he did not develop the disease. Later Jenner used material for vaccination from cowpox lesions and successfully vaccinated 23 persons. As the material he used was obtained from cow (latin vacca), latter the term vaccination was used by Louis Pasteur for inoculation against disease.

In 1884, one of Pasteur’s associates, Charles Chamberland, found that bacteria can not pass through porcelain filters, while agent responsible for rabies (a disease which is transferred to human by bites of rabid dogs, foxes, cats, bats and other animals) can pass through these filters. As in those days the word virus was loosely used to describe any toxic substance that caused disease, those unseen filterable agents of disease were described as filterable viruses. In 1892, Ivanowski discovered that the agent which caused tobacco mosaic disease was filterable. He obtained bacteria free filtrate from ground up infected plants and placed it on healthy leaves of tobacco. He observed that filtrate produced the disease in healthy plants. After that, presence of similar filter-passing, ultramicroscopic agents was seen in the victims of many diseses, including foot and mouth disease (1898) and yellow fever (1901).

Bacteriophages, viruses that infect bacteria were discovered independently by Twort in 1915 and D’Herelle in 1917. Twort observed that bacterial colonies sometimes undergo Lysis (dissolved and disappeared) and that this Lysis can be transferred from one colony to other. Even highly diluted material from Lysed colony can transfer the Lytic effect. However, heating the filtrate destroyed its Lytic property. From these observations he concluded that Lytic agent might be a virus. D’Herelle rediscovered this phenomena in 1917 and used the word bacteriophages meaning “bacteria eater”.

 The filterable agents were first purified in 1935, when Stanley was successful in crystallizing the tobacco mosaic virus. Chemical analysis of these particles showed that they contained only nucleic acid and protein. This suggested that, unlike other forms, viruses are of simple chemical composition.

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