From The Collaborative International Dictionary of English v.0.48:

Virus \Vi"rus\, n. [L., a slimy liquid, a poisonous liquid,
   poison, stench; akin to Gr. ? poison, Skr. visha. Cf.
   Wizen, v. i.]
   1. (Med.) Contagious or poisonous matter, as of specific
      ulcers, the bite of snakes, etc.; -- applied to organic
      poisons. [Archaic]
      [1913 Webster +PJC]

   2. the causative agent of a disease, . [obsolescent]

   3. any of numerous submicroscopic complex organic objects
      which have genetic material and may be considered as
      living organisms but have no proper cell membrane, and
      thus cannot by themselves perform metabolic processes,
      requiring entry into a host cell in order to multiply. The
      simplest viruses have no lipid envelope and may be
      considered as complex aggregates of molecules, sometimes
      only a nucleic acid (DNA or RNA) and a coat protein. They
      are sometimes viewed as being on the borderline between
      living and nonliving objects. They are smaller than living
      cells in size, usually between 20 and 300 nm; thus they
      pass through standard filters, and were previously
      referred to as filterable virus. The manifestations of
      disease caused by multiplication of viruses in cells may
      be due to destruction of the cells caused by subversion of
      the cellular metabolic processes by the virus, or by
      synthesis of a virus-specific toxin. Viruses may infect
      animals, plants, or microorganisms; those infecting
      bacteria are also called bacteriophages. Certain
      bacteriophages may be non-destructive and benign in the
      host; -- see bacteriophage.
      [1913 Webster +PJC]

   4. Fig.: Any morbid corrupting quality in intellectual or
      moral conditions; something that poisons the mind or the
      soul; as, the virus of obscene books.
      [1913 Webster]

   5. (Computers) a program or segment of program code that may
      make copies of itself (replicate), attach itself to other
      programs, and perform unwanted actions within a computer;
      also called computer virus or virus program. Such
      programs are almost always introduced into a computer
      without the knowledge or assent of its owner, and are
      often malicious, causing destructive actions such as
      erasing data on disk, but sometime only annoying, causing
      peculiar objects to appear on the display. The form of
      sociopathic mental disease that causes a programmer to
      write such a program has not yet been given a name.
      Compare trojan horse[3].
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