Virus (kingdom)

Scope note: Extremely small, metabolically inert microorganisms composed of a short strand of either DNA or RNA within a protein coat, and, in some, a surrounding envelope; they replicate only within the cells of living hosts, primarily plants, animals, and bacteria, taking over the nucleus and directing it to produce viral proteins and DNA or RNA. They frequently cause disease in their hosts. Viruses occupy a special taxonomic position: they are not plants, animals, or prokaryotic bacteria (single-cell organisms without defined nuclei), and they are generally placed in their own kingdom. In fact, viruses should not even be considered organisms, in the strictest sense, because they are not free-living, meaning they cannot reproduce and carry on metabolic processes without a host cell.

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