Diseases Caused by Viruses

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.

Nature of viruses

Viruses vary in their size ranging from 20 nm (200 Ao) to 350 nm (3,500 Ao) and in shapes from spherical to bullet, rod, brick or filamentous forms. Some of the viruses are exceedingly fragile and are easily destroyed by common physical and chemical agents such as heat, sunlight, alkalies and disinfectants. They can be stored at ultra- low temperatures ranging from-70o to-196o C for several years without any loss in their infectivity or other properties. They can also be lyophilized and their stability can be improved further by the addition of cryopreservatives such as 5-10% dimethyl sulphoxide or 50% glycerol.A virus particle, also called  virion, consists of a nucleic acid core with its strands (20 to 25o A in diametre), a protein coat known as  capsid  and in a few viruses, an envelope derived from the nuclear or cytoplasmic membrane of the host cells. The nucleic acid which is either DNA or RNA is single or double stranded and is formed by 3 or 4 genes in smaller viruses to several hundreds of genes in larger viruses. The proportion of nucleic acid in a virion varies from 1-50%. The capsid enclosing the nucleic acid is made up of number of subunits called capsomers which are connected together by chemical bonds. There may be 1 or 2 capsids. The capsids are icosahedral or helical depending upon the structural symmetry of the virions. The envelope, when present, contains lipid layers and proteins that are synthesized as specified by viral genes contributing to the antigenic specificity. The structural chemistry of the virions is studied by electron microscopy, X-ray crystallography or by neutron diffraction technologies. Based on the physico-chemical characteristics and the kind of host and of vectors involved, viruses have been classified into 8-DNA and 15-RNA virus families.

Recent advances in nucleic acid and protein biochemistry have provided a detailed insight into the molecular aspects of virus gene expression. Some of the recent techniques, such as, gene cloning and sequencing, electron microscopy and nucleic acid hybridization have revolutionized the concept of viral pathogenesis and have added to our knowledge of understanding the viruses and their nature and have also provided means for development of accurate diagnostic tests, diagnostic probes and
more effective vaccines. The recombinant DNA and hybridoma technologies are finding more and more application in modern virology.