retrovirus (n.)

late 14c., "poisonous substance" (a sense now archaic), from Latin virus "poison, sap of plants, slimy liquid, a potent juice," from Proto-Italic *weis-o-(s-) "poison," which is 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" (source also 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").

The meaning "agent that causes infectious disease" emerged by 1790s gradually out of the earlier use in reference to venereal disease (by 1728); the modern scientific use dates to the 1880s. The computer sense is from 1972.

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]

word-forming element of Latin origin meaning "backwards; behind," from Latin retro (prep.) "backward, back, behind," usually in reference to place or position, rarely of time, "formerly, in the past," probably originally the ablative form of *reteros, based on re- "back" (see re-).

L. retro stands to re- as intro, "in, within"; to in, "in," and as citro, "hither," stands to cis, "on this side." [Klein]

Common in combinations in post-classical Latin (the classical equivalent was post-). Active in English as a word-forming element from mid-20c.