In the early days of microbiology, the disease producing submicroscopic agents were termed as 'filterable viruses' because of their ability to pass through conventional filters which retain bacteria. The existence of viruses was first observed in 1892 in the mosaic disease of the tobacco plants by Beijerinck and Ivanovski. That the foot-and- mouth disease of cattle is caused by a virus was discovered in 1898 by Loefler and Frosch. Since then a large number of viruses from plants, animals and human beings have been recognized for their involvement as etiological agents of disease.