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.

Informal . a viral disease.

a corrupting influence on morals or the intellect; poison.

a segment of self-replicating code planted illegally in a computer program, often to damage or shut down a system or network.

noun

ailment , bacillus , disease , germ , illness , infection , microbe , microorganism , pathogen , sickness , bane , canker , contagion , toxin , venom , bug , flu , poison