After the discovery by Louis Pasteur and Robert Koch that infectious diseases were caused by minute living organisms or 'germs', it was expected that the germs for all infectious diseases would be discovered. However, bacteriological techniques failed to demonstrate the causative organisms for many diseases like measles, small pox, rabies and mumps.
The Russian botanist Ivanovski (lwanowsky) (1892) was the first to give clear cut evidence of a virus. Mayer (1886) had demonstrated that when juice from tobacco plants infected with the 'mosaic' disease was injected into healthy plants, it reproduced the mosaic disease. Boiling the juice destroyed the infectivity. Mayer thought that, the causative agent was a bacterium. Inoculation of tobacco plants with a variety of bacteria, however, failed to produce the tobacco, mosaic disease.
Ivanovski confirmed the observations of Mayer, and also made another very important one. Even after filtering through the finest bacterial filters, the juice still remained infective. Ivanovski concluded that the agent was smaller than any known bacterium, but he still considered it to be a bacterium. This agent was later called a virus. Bacteriophages ( viruses that parasitise bacteria) were discovered by the French scientist d'Herelle (1917), who found that some agent was destroying his cultures of bacilli.
The Russian botanist Ivanovski (lwanowsky) (1892) was the first to give clear cut evidence of a virus. Mayer (1886) had demonstrated that when juice from tobacco plants infected with the 'mosaic' disease was injected into healthy plants, it reproduced the mosaic disease. Boiling the juice destroyed the infectivity. Mayer thought that, the causative agent was a bacterium. Inoculation of tobacco plants with a variety of bacteria, however, failed to produce the tobacco, mosaic disease.
Ivanovski confirmed the observations of Mayer, and also made another very important one. Even after filtering through the finest bacterial filters, the juice still remained infective. Ivanovski concluded that the agent was smaller than any known bacterium, but he still considered it to be a bacterium. This agent was later called a virus. Bacteriophages ( viruses that parasitise bacteria) were discovered by the French scientist d'Herelle (1917), who found that some agent was destroying his cultures of bacilli.
Schelsinger (1933) was the first to determine the composition of a virus. He showed that a bacteriophage consists of only protein and DNA.
In 1935 Stanley crystallized the virus causing tobacco mosaic disease, and demonstrated that the crystals retained their infectivity when inoculated into healthy plants. He thus showed that viruses were not like typical cells.
In 1952 Hershey and Chase studied - the T2 bacteriophage and demonstrated that (1) the genetic information is carried in the phage DNA, and that (2) infection is the result of penetration of viral DNA into cells.
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