means A tiny infectious agent that hijacks living cells to copy itself, or by extension a malicious self-replicating program that does the same to your computer.

from Straight from Latin virus, meaning 'poison,' 'slimy liquid,' or 'venom' — the stuff that oozes and harms. For centuries in English it just meant a poisonous substance; only in the late 19th century, as scientists hunted infectious agents too small to see or filter, did it narrow to the microscopic culprit we now dread. The computer sense is a 20th-century borrowing, naming code that spreads like an infection.