History of Viruses and Virology

French-Canadian microbiologist Felix d'Herelle of the Institute Pasteur in Paris and later of the Université de Montreal discovered viruses in bacteria in 1917. Calling them bacteriophages (literally, "bacteria eaters"), d'Herelle inferred their existence when he found that infectious agents passed through filters with pores too small to admit passage by bacteria. His work followed on that of British bacteriologist F. W. Twort, who found Staphylococcus bacteria that were infected. Although subvisible agents causing infection in plants and animals had previously been known and named viruses or virus pathogens, not until the work of Twort and d'Herelle was it understood that they required host cells to reproduce.

Viruses come in two basic varieties, DNA viruses and RNA viruses. A DNA virus transcribes its DNA into RNA, which uses the host cell to make viral proteins instead of the cell's own proteins. Interfering with the normal functioning of cells, these new proteins at worst can destroy the cells whose machinery they borrow to promote themselves. When a DNA virus attacks a eukaryotic cell, it uses the mitochondria of the host cell to gain energy to rearrange its amino acids into the new pro teins, some of which are enzymes that cut and splice pieces of the virus's protein and DNA to make new proteins and DNA. In extreme cases the entire cell is converted into viruses and the viruses burst out, rupturing and destroying the host cell. This process is called lysis. Less violent DNA viruses are not as dangerous to their hosts. For example, thirty different sorts of adenoviruses are commonly found in humans, and their only effect is the minor respiratory aliments we call colds; these same viruses, however, can produce cancer if transmitted to rodents—a less stable environment with which they share no evolutionary history. DNA viruses are further classified into whether their genome is single- or double-stranded.

RNA viruses do not even possess their own genes. They are, rather, tiny chemical bundles that mimic the host's messenger RNA, the RNA that translates the "code" (nucleotide sequence) of DNA into the amino acid sequence of proteins in live cells. Intercepting cell reproduction at the protein-building level, RNA viruses are responsible for yellow fever, Colorado tick fever, mumps, AIDS, and foot-and-mouth disease in livestock. Tumor-forming or oncogenic viruses are also known from this group.

The Bittner virus is an RNA virus that causes cancer in mice; the virus, transmitted through the mother's milk, may reach concentrations as high as 50 billion viral particles per drop. Such oncogenic viruses were first discovered in chickens with symptoms of the blood disease leukemia. At first most of the cells invaded by a tumor-forming virus die. The remaining cells, however, turn cancerous. They reproduce without respect for the normal taboo on tissue growth. The growth of the cancer cells, ultimately fatal to the animal body as a whole, in the short term helps replicate the virus by forming tumors. RNA viruses are subdivided into two types of single-strand RNA and one type of double-strand RNA. Still other viruses, retroviruses (such as HIV) and hepadnaviruses, are known and classified according to the intricacies of their genetic integration into host cells. Exposed to ultraviolet radiation, some bacteria lyse, releasing hoards of viruses. Because they are small and mobile, viruses play a role in evolution, by spreading bits of DNA and RNA from one species to another; although more likely to cause malfunction and disease, occasionally such transferences may be useful. Evolutionar-ily, the first viruses are thought to have been pieces of nucleic acid that became dislodged, perhaps under the influence of solar ultraviolet radiation, from the genes of bacteria. The lack of an ozone layer on the early earth exposed cells to relatively high levels of radiation. Because of their role in disease, the study of viruses, virology, is a well-funded and vibrant subdiscipline of medical microbiology.

—Dorion Sagan and Lynn Margulis

See also: Bacteria; Classification, Biological; Coevo-lution; Five Kingdoms of Nature; Microbiology; Natural Selection; Population, Human, Curbs to Growth; Protoctists

Bibliography

Dixon, Bernard. 1994. Power Unseen: How Microbes Rule the World. Oxford: W. H. Freeman; Guerrero, Ricardo. 2000. "The Biosphere Concept and Index." In Encyclopedia of the Biosphere, edited by Ramon Folch. Vol. 11, pp. 398-399. Detroit: Gale Group; Sagan, Dorion, and Lynn Margulis. 1993. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Dubuque, IA: Kendall/Hunt.

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