I knew that virus originally meant simply 'poison' and guessed it came from Latin, but I didn't know its full history. It's attested in late Middle English from about 1400, with the sense of 'pus', from the Latin virus, 'poison' or 'poisonous secretion'.
The PIE root this derives from is *wisu-, or *uisos-, which has cognates in various languages, including English viscous, from Latin viscum, 'sticky flow' or 'sticky substance', which entered the language at about the same time as virus, so these two words are doublets.
A particularly interesting modern descendent of the PIE root is Ancient Greek ιός, ios, 'poison', which has survived into Modern Greek as the word for virus.
There might seem a bit of a gap between virus and ios, but the explanation is that Greek at one time had a 'w' sound, written as digamma ϝ , but this had been lost by the Classical era. There are quite a few Greek words that formerly began with 'w' but later lost it. A fine example is woikos, 'home' which became oikos (think ecology, economy). In Latin the same root gave vicus (think vicinity) and vicus was borrowed into Germanic as wick (think Warwick).
Footnote on digamma. It was so called because it looked like one gamma Γ sitting atop another Γ. The Greek alphabet was carried to Italy and adopted by the Etruscans and from there by the Romans. However the Romans wrote 'v' for both the sounds 'w' and 'v' so they were free to recycle the redundant ϝ as the consonant that has come down to us as ef.
PS while I was looking stuff up on Google 'AI Overview" made a joke. Not a brilliant one, but definitely a joke. The Archaic Greek for poison was *wīsos but as you recall the 'w' sound got dropped and then so did the intervocalic s, leaving just ios.
The AI quipped
If that internal s hadn't vanished, we might be calling viruses "wisos-logy" instead of virology today!
(The language is a bit muddled in the middle portion but you get the idea.)