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The Collaborative International Dictionary of English v.0.48 (gcide)

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]
[PJC]

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]}.
[PJC]