Virus, the smallest form of life (20-300 nanometres), is structurally and functionally unique.
Structure and Function
The mature virus organism (virion) consists of viral genes contained in a protective protein shell (a capsid). Viral genes may be deoxyribonucleic acid (DNA) or ribonucleic acid (RNA); genetic RNA is unique to viruses. RNA viruses include enteroviruses (eg, causing poliomyelitis), rhinoviruses (common cold), rhabdoviruses (rabies), paramyxoviruses (measles), orthomyxoviruses (influenza) and almost all plant viruses.
DNA viruses include papavoviruses (warts), adenoviruses (acute respiratory disease), herpesviruses (cold sore, infectious mononucleosis, chicken pox), poxviruses (smallpox, cowpox), hepatitis B, and many viruses that infect bacteria (bacteriophages) and insects.
Similarly, plant-specific viruses (discovered by Russian botanist Dimitri Ivanovsky in 1892) and animal-specific viruses (discovered by Friedrich Loeffler and Paul Frosch in 1898) have preferred host cells. Such preferences may stem from the origins of viruses, which are thought by some theorists to be degenerate, subcellular, self-replicating particles.
Viruses infect cells by fusion, endocytosis or injection. In viruses of a certain structure, the viral envelope fuses with the cell membrane of a host cell and the virus genome it contains is introduced into the cell. In endocytosis the host cell engulfs the virus, engulfment being triggered by contact with a viral particle, and the nucleic material is released from the capsid.
Cells undergoing a lytic virus infection frequently have their own synthetic processes inhibited but produce hundreds of identical copies of the infecting virion. The replicative strategies employed by RNA viruses are diverse and complex, depending upon whether the virus genome can function directly as messenger RNA, which instructs host cell ribosomes (cell organelles through which protein synthesis occurs) to make viral enzymes and proteins, or whether messenger RNA must be synthesized on the viral genetic template, before protein and enzyme synthesis can proceed.
Viruses cause major diseases in plants and animals, but there are few effective methods for cure. Plant breeding may yield strains resistant to viruses, but this avenue of approach is unavailable to humans.
More successful efforts have been directed towards the development of vaccines for preventing viral disease. Properly administered, such vaccines have resulted in the worldwide eradication of smallpox and the virtual disappearance of poliomyelitis, measles and rubella from specific populations.