An Etymological Dictionary of Astronomy and Astrophysics

1. An ultramicroscopic (20 to 300 nm in diameter), metabolically inert, infectious agent that replicates only within the cells of living hosts, mainly bacteria, plants, and animals: composed of an RNA or DNA core, a protein coat, and, in more complex types, a surrounding envelope (Dictionary.com).
   

2. → computer virus.

See also: From L. virus “poison, sap of plants, slimy liquid, a potent juice,” ultimately from PIE *ueis- “fluidity, slime, poison;” cf. Pers. bīš “a poisonous plant;” Mid.Pers. wiš “poison;” Av. viš-, viša- “poison;” Skt. visa- “venom, poison, poisonous;” L. viscum “sticky substance, birdlime;” Gk. ios “poison,” ixos “mistletoe, birdlime;” O.C.S. višnja “cherry;” O.Irish fi “poison;” Welsh gwy “poison.”