late 14c., "venomous substance," from Latin virus "poison, sap of plants, slimy liquid, a potent juice," probably from PIE root *weis- "to melt away, to flow," used of foul or malodorous fluids, with specialization in some languages to "poisonous fluid" (cognates: Sanskrit visam "poison," visah "poisonous;" Avestan vish- "poison;" Latin viscum "sticky substance, birdlime;" Greek ios "poison," ixos "mistletoe, birdlime;" Old Church Slavonic višnja "cherry;" Old Irish fi "poison;" Welsh gwy "fluid, water," gwyar "blood"). Main modern meaning "agent that causes infectious disease" first recorded 1728 (in reference to venereal disease). The computer sense is from 1972.