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  Virology, in the life sciences, as its Greek name suggests, is the study of the structure and function of submicroscopic particles, collectively termed viruses. The existence of viruses as agents of infection was first demonstrated in 1892 by Dmitry Ivanovsky when he used a porcelain filter to show that tobacco mosaic disease could be transmitted between plants by a bacteria-free filtrate. At the turn of the century, foot and mouth disease and yellow fever were shown to be caused by filterable viruses, but the viruses themselves could not be visualized until the development of the electron microscope in the 1940s. Prior to this, the tobacco mosaic virus had been crystallized in 1935 from thousands of pure particles; this development enabled the first work on the structure of virus particles by X-ray crystallization.

It is now known that viruses are simple particles which do not grow and which can only replicate by parasitizing a living cell and redirecting its activity to produce many virus copies. A viral particle consists of a package of nucleic acids surrounded by a coat of protein, which protects the nucleic acids from the environment. The virus cannot be considered alive outside its host cell; it is merely an inert dispersal stage. The core of nucleic acids carries the information necessary to commandeer the cell and to instruct it to replicate the virus; for this reason most viruses infect only specific cell types. This form of parasitism must have a cost to the host cell and, where changes are observed, the virus is said to be cytopathogenic. It is not clear how many viruses infect cells without causing obvious pathology, though these may be in the majority. The protein coat is antigenic: it is this which stimulates immunity after exposure to the virus, or vaccination with an attenuated form. Many viral diseases have been effectively controlled by vaccination; smallpox has been eradicated and poliomyelitis is now extremely uncommon. Others such as the common cold and influenza have proved difficult to control because there are many types of virus responsible, all of which have different surface coat proteins.

Virology has advanced rapidly over the past decade as a result of the intense international effort to control the human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS) in humans. Despite this, there is no vaccine or successful treatment so far.

Recent evidence suggests that a class of infective agents may exist which have no nucleic acids; these largely hypothetical agents called prions, may be the cause of diseases such as bovine spongiform encephalopathy (BSE). The possibility of replication in living cells without nucleic acids is a radical concept in biology; in this case the material which is responsible for carrying genetic information appears to be protein. The implication of this is that the prions, once within a host cell, must use protein to encode a nucleic acid sequence so that the cell may replicate the prion. If this is the case, then an important assumption of biology, that information flow between nucleic acids (DNA/RNA) and amino acids (protein) is unidirectional, is no longer valid. Beside the pathogenic viruses, there is another group of great importance to humans. These are the bacteriophages, which parasitize bacterial cells, inserting their genetic information and thus altering the genotype of the organism (transduction). This feature has been turned to advantage by molecular biologists who use such viruses in genetic engineering. RB

See also aetiology; microbiology; parasitism; parasitology.Further reading Scientific American, Micro-organisms from Smallpox to Lyme Disease.



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