virus

UK: /ˈvaɪərəs/ US: /ˈvaɪrəs/ ,(vī′rəs)

Inflections of 'virus' (n): npl: viruses

WordReference Random House Learner's Dictionary of American English © 2026

vi•rus /ˈvaɪrəs/   n. [countable] , pl. -rus•es. 

1. Microbiology a very small living thing causing infection, which reproduces only within the cells of living hosts, mainly bacteria, plants, and animals.
2. Pathology, Informal Terms a disease caused by a virus:
   was ill from a virus.
3. a corrupting idea or thought or a harmful influence on morals or the intellect.
4. Computing a part of a computer program that is planted illegally in another program, often to damage or shut down a system or network.

WordReference Random House Unabridged Dictionary of American English © 2026

vi•rus  (vī′rəs), n., pl. -rus•es. 

1. Microbiology an ultramicroscopic (20 to 300 nm in diameter), metabolically inert, infectious agent that replicates only within the cells of living hosts, mainly bacteria, plants, and animals: composed of an RNA or DNA core, a protein coat, and, in more complex types, a surrounding envelope.
2. Pathology, Informal Terms [Informal.] a viral disease.
3. a corrupting influence on morals or the intellect;
   poison.
4. Computing a segment of self-replicating code planted illegally in a computer program, often to damage or shut down a system or network.

• Latin vīrus slime, poison; akin to ooze²
• 1590–1600

vi ′rus•like′, adj. 

Collins Concise English Dictionary © HarperCollins Publishers::

virus / ˈvaɪrəs/ noun ( -ruses)

1. any of a group of submicroscopic entities consisting of a single nucleic acid chain surrounded by a protein coat and capable of replication only within the cells of living organisms: many are pathogenic
2. a disease caused by a virus
3. any corrupting or infecting influence
4. an unauthorized program that inserts itself into a computer system and then propagates itself to other computers via networks or disks; when activated it interferes with the operation of the computer

Etymology: 16th Century: from Latin: slime, poisonous liquid; related to Old English wāse marsh, Greek ios poison