Origin and history of virus

late 14c., "poisonous substance" (a sense now archaic), originally in pathology "pus, thin fluid discharged from a wound or sore;" from Latin virus "poison, poisonous liquid, sap of plants, slimy liquid, a potent juice," from Proto-Italic *weis-o-(s-) "poison."

This is reconstructed to be probably from a PIE root *ueis-, perhaps originally meaning "to melt away, to flow," used of foul or malodorous fluids, but with specialization in some languages to "poisonous fluid."

VIRUS (among Physicians) a kind of watery stinking Matter, which issues out of Ulcers, being endued with eating and malignant Qualities. [Bailey's dictionary, 1770]

By 1790s the scientific meaning had focused to "contagium of an infectious disease, agent produced in the body of the infected and capable of infecting others with the same disease," gradually from earlier use in reference to the contagious pus of venereal disease (by 1728). The modern scientific use in reference to disease-causing submicroscopic organisms dates to the 1870s and the word was applied to them when they began to be discovered late 1890s. The extended sense in reference to computers is by 1972.

The PIE root also is reconstructed as the source of Sanskrit visam "venom, 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 "poison."