<p><strong>Noun</strong></p><ul><li>(plural viruses) an ultramicroscopic (20 to 300 nanometers 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.</li><li>a viral disease: He stayed home sick with a virus for nearly two weeks.</li><li>a corrupting influence on morals or the intellect; poison.</li><li>a segment of self-replicating code planted illegally in a computer program, often to damage or shut down a system or network.</li></ul>