Virurses are ultramicroscopic, non-cellular, highly infectious agents that multiply only introcellularly inside the living host cells without involving growth and division. Outside the living cells they behave as inert particles. They are nucleoproteins having one or more nucleic acid molecules either DNA or RNA enclosed in a coat of protein or lipoprotein.
The name virus that means venom or poisonous fluid was given by Dmitri Ivanowsky (1892) recognised certain microbes as casual organism of mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria proof filters. M.W. Beijrinck (1898) demonstrated that the extract of the injected plant of tobacco could cause infection in healthy plants and called the fluid contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely proteins. Viruses are considered as obligate parasites.
Viruses are highly pathogenic and cause infectious diseases in animals and plants. They cause infection in the host cells by means of nucleic acids,
- In general viruses that infect plants have single stranded RNA
- viruses that infect animals have either single or double stranded RNA or double stranded DNA and
- Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.
Some viruses possess an outer thin loose covering called envelop composed of proteins, lipids and carbohydrates. The smaller subunits of envelope are called peptomere. The protein coat of viruses is called capsid made of small subunits called capsomeres protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.
Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also caused by a virus Human Immunodeficiency Virus (HIV). In plants the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearning, dwarfing and stunted growth.
