n. [ L., a slimy liquid, a poisonous liquid, poison, stench; akin to Gr. &unr_; 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 ]