The word 'virus' entered English directly from Latin, where it originally meant 'poison' or 'slimy liquid'. This Latin term itself is of uncertain deeper origin, though it may be related to the Sanskrit word 'viṣa' (poison) and the Greek 'ios' (poison, arrow tip), suggesting a very ancient Proto-Indo-European root for a harmful substance. The modern scientific meaning, referring to a submicroscopic infectious agent, was established in the late 19th century, drawing a direct metaphorical link from the ancient concept of a poison to a biological entity that invades and harms a host. The term was later adopted in computing during the 1970s by analogy, describing a program that replicates itself and corrupts a system, much like its biological counterpart.

The word 'virus' is a simplex loanword from Latin. It does not contain separable, meaningful morphemes in its modern English spelling.