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Medical Term:


Pronunciation: vi'rus


  1. Formerly, the specific agent of an infectious disease.
  2. Specifically, a term for a group of infectious agents, which, with few exceptions, are capable of passing through fine filters that retain most bacteria, are usually not visible through the light microscope, lack independent metabolism, and are incapable of growth or reproduction apart from living cells. They have a prokaryotic genetic apparatus but differ sharply from bacteria in other respects. The complete particle usually contains either DNA or RNA, not both, and is usually covered by a protein shell or capsid that protects the nucleic acid. They range in size from 15 to several hundred nanometers. Classification of viruses depends on physiochemical characteristics of virions as well as on mode of transmission, host range, symptomatology, and other factors. For viruses not listed below, see the specific name.
  3. Relating to or caused by a virus, as a viral disease.
  4. Obsolete usage. From before the development of bacteriology, any agent thought to cause disease, including a chemical substance such as an enzyme (“ferment”) similar to snake venom; synonymous at that time with “poison.”

Synonym(s): filtrable virus

[L. poison]

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