vi?rus (?va? r?s) n., pl. -rus?es. 1. 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. a disease caused by a virus. 3. a corrupting influence on morals or the intellect; poison. 4. a segment of self-replicating code planted illegally in a computer program, often to damage or shut down a system or network. [1590?1600;