Virology is the study of viruses – submicroscopic, parasitic particles of genetic material contained in a protein coat – and virus-like agents. A major branch of virology is virus classification. Viruses can be classified according to the host cell they infect: animal viruses, plant viruses, fungal viruses, and bacteriophages (viruses infecting bacterium, which include the most complex viruses). The most useful and most widely used classification system distinguishes viruses according to the type of nucleic acid they use as genetic material and the viral replication method they employ to coax host cells into producing more viruses:
DNA viruses (divided into double-stranded DNA viruses and single-stranded DNA viruses), RNA viruses (divided into positive-sense single-stranded RNA viruses, negative-sense single-stranded RNA viruses and the much less common double-stranded RNA viruses), reverse transcribing viruses (double-stranded reverse-transcribing DNA viruses and single-stranded reverse-transcribing RNA viruses including retroviruses). The latest report by the International Committee on Taxonomy of Viruses (2005) lists 5450 viruses, organized in over 2,000 species, 287 genera, 73 families and 3 orders. Virologists also study subviral particles, infectious entities notably smaller and simpler than viruses:
Some viruses, known as oncoviruses, contribute to the development of certain forms of cancer. The best studied example is the association between Human papillomavirus and cervical cancer: almost all cases of cervical cancer are caused by certain strains of this sexually transmitted virus. Another example is the association of infection with hepatitis B and hepatitis C viruses and liver cancer. The study of the manner in which viruses cause disease is viral pathogenesis. The degree to which a virus causes disease is its virulence. When the immune system of a vertebrate encounters a virus, it may produce specific antibodies which bind to the virus and neutralize its infectivity or mark it for destruction. A second defense of vertebrates against viruses, cell-mediated immunity, involves immune cells known as T cells: the body's cells constantly display short fragments of their proteins on the cell's surface, and if a T cell recognizes a suspicious viral fragment there, the host cell is destroyed and the virus-specific T-cells proliferate. , most likely evolved as a defense against viruses. Today it is believed that most viruses are relatively benign in their natural hosts; some viral infection might even be beneficial to the host. For example, viruses that cause serious influenza in humans probably have pigs or birds as their natural host, and HIV is thought to derive from the benign non-human primate virus SIV. Bacteriophages, the viruses which infect bacteria, can be relatively easily grown as viral plaques on bacterial cultures. Certain genetic elements derived from viruses, such as highly effective promoters, are commonly used in molecular biology research today. Growing animal viruses outside of the living host animal is more difficult. This approach of using viruses as gene vectors is being pursued in the gene therapy of genetic diseases. An obvious problem to be overcome in viral gene therapy is the rejection of the transforming virus by the immune system. Oncolytic viruses are viruses that preferably infect cancer cells. A new application of genetically engineered viruses in nanotechnology was recently described; see the uses of viruses in material science and nanotechnology. The word virus appeared in 1599 and originally meant "venom". The nature of viruses however was not clear to these researchers. In 1903 it was suggested for the first time that transduction by viruses might cause cancer. The virus was later called Rous sarcoma virus 1 and understood to be a retrovirus. While plant viruses and bacteriophages can be grown comparatively easily, animal viruses normally require a living host animal, which complicates their study immensely. In 1937, Max Theiler managed to grow the yellow fever virus in chicken eggs and produced a vaccine from an attenuated virus strain; this vaccine saved millions of lives and is still being used today. The first virus that could be crystalized and whose structure could therefore be elucidated in detail was tobacco mosaic virus (TMV), the virus that had been studied earlier by Ivanovski and Beijerink. Until that time, it was thought that these viruses carried certain genes called oncogenes which, when inserted into the host's genome, would cause cancer. Michael Bishop and Harold Varmus showed that the oncogene of Rous sarcoma virus is in fact not specific to the virus but is contained in the genome of healthy animals of many species. 1976 saw the first recorded outbreak of Ebola virus disease, a highly lethal virally transmitted disease. Subsequent tremendous research efforts turned HIV into the best studied virus. Assembling the 7741-base genome from scratch, starting with the virus's published RNA sequence, took about two years. The technique uses modified retroviruses to transform the cells; this is a potential problem for human therapy since these viruses integrate their genes at a random location in the host's genome, which can interrupt other genes and potentially causes cancer. In 2008, Sputnik virophage was described, the first known virophage: it uses the machinery of a helper virus to reproduce and inhibits reproduction of that helper virus.
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