DISCOVERY OF VIRUSES

The infections caused by viruses such as Rabies, Rinderpest and Smallpox have been

known and feared since the dawn of history but the nature of their aetiology remained a

myth. The discovery and study of viruses started sometimes in the late nineteenth century

much after those of bacteria and fungi have been established.

In 1892, Dmitri Ivanovsky, a Russian scientist, reported that tobacco mosaic disease

could be transmitted from a diseased plant to a healthy one using filtered leaf extract

from the diseased plant as inoculums. Ivanovsky used Chamberland filter (designed in

Pasteur laboratory by Charles Chamberland) to filter leaf extract from tobacco plant

infected with tobacco mosaic. The filtrate was used as inoculums to infect a healthy

tobacco plant. Ivanosky observed that the inoculated plant developed the disease tobacco

mosaic. He then reasoned that the agent of tobacco mosaic couldn’t have been a

bacterium since Chamberland filter could hold back even the smallest bacterium. The

agent must be smaller than bacteria to have passed through the filter. In 1898, Martinus

Beijerinck (a Dutch) unaware of the discovery by Ivanovsky also demonstrated the

filterability of the agent of tobacco mosaic disease. He also showed that the disease

couldn’t have been due to a toxin because the filtered sap from infected plant could be

used for serial transmission of the disease without loss of potency. The filterable diseasecausing

agent was termed ‘virus’ meaning ‘poisonous fluid’.

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In 1898, Loeffler and Frosch showed that Foot-and-Mouth-Disease was caused by a

filterable agent. In 1901, Walter Reed and his team identified Yellow Fever Virus as a

filterable pathogen in humans.