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Viruses are similar to, but not considered, living organisms. To be considered "living", an organism must be able to reproduce, either asexually or sexually. Viruses, however, are only able to replicate themselves by commandeering the reproductive apparatus of the cells of living beings and making them reproduce the virus's genetic structure instead. Thus, a virus cannot function or reproduce outside of another being's cell, thereby being totally dependent on a host cell in order to survive.〔N.J. Dimmock et al. "Introduction to Modern Virology, 6th edition." Blackwell Publishing, 2007.〕 Most viruses are species specific, and related viruses typically only infect a narrow range of plants, animals, bacteria, or fungi. ==Exposure of host== Usually viral infection occurs when a virus enters the host, either: * through a physical breach (a cut in the skin) * direct inoculation (e.g.mosquito bite〔Bureau for Public Health Division of Surveillance and Disease Control. "Take precautions against mosquitos, dead birds to prevent West Nile, encephalitis viruses." W V Med J. 2005 Mar-Apr;101(2):90.〕) * direct infection of the surface itself (inhalation of the virus into trachea〔Quan FS, Compans RW, Nguyen HH, Kang SM. "Induction of Heterosubtypic Immunity to Influenza Virus by Intranasal Immunization." J Virol. 2007 Nov 21〕) It is usually only after a virus enters a host that it can gain access to possible susceptible cells. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「viral life cycle」の詳細全文を読む スポンサード リンク
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