The word 'virus' has undergone two major semantic transformations in its English career, each reflecting a revolution in human understanding. It comes from Latin 'vīrus,' meaning 'poison,' 'venom,' or 'slimy liquid,' from PIE *weys- (to flow, to ooze, particularly something unpleasant or toxic).
The PIE root *weys- produced cognates in several branches of the family. Sanskrit 'viṣá-' (poison — the origin of the word 'Vishnu' according to some etymologies, though this is disputed) and Avestan 'vīša' (poison) represent the Indo-Iranian branch. Greek 'iós' (ἰός, poison, venom, and also 'rust' — rust conceived as a slow poison attacking metal) represents the Hellenic branch. Old Irish 'fí' (poison) represents the Celtic branch. The semantic core across
In Latin, 'vīrus' was a neuter noun of the second declension, but it was almost always used as a mass noun — it had no standard plural form (like English 'water' or 'mud'). The modern English plural 'viruses' is an English formation; the sometimes-seen 'virii' or 'viri' are hypercorrections with no basis in Latin grammar.
The word entered English around 1590 meaning 'venom' or 'poison' in a general sense. Through the seventeenth and eighteenth centuries, it was used loosely for any infectious or poisonous substance thought to cause disease.
The first transformation came in 1892, when the Russian botanist Dmitri Ivanovsky demonstrated that the agent causing tobacco mosaic disease could pass through a porcelain filter that trapped all known bacteria. These 'filterable viruses' (agents too small to be bacteria) were a new category of pathogen. By the early twentieth century, 'virus' had been specialized to mean this newly discovered class of sub-microscopic infectious agents — obligate intracellular parasites consisting of genetic material wrapped in protein.
The second transformation came in 1972, when the science fiction writer David Gerrold used 'virus' to describe a self-replicating computer program in his novel 'When HARLIE Was One.' Fred Cohen formalized the concept in his 1986 PhD thesis, defining a computer virus as 'a program that can infect other programs by modifying them to include a possibly evolved copy of itself.' The biological metaphor was deliberate and powerful: like biological viruses, computer viruses reproduce by hijacking the machinery of their host, spread through contact, and can cause devastating harm.
The adjective 'viral' has undergone its own transformation. 'Going viral' (spreading rapidly through a population or across the internet) fuses the biological and digital senses of the word, treating the rapid propagation of information as structurally analogous to an epidemic. The word 'virulent' (extremely harmful) preserves the original Latin sense of 'poisonous.'