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Noun, pl. viruses

1. A small infectious agent that can replicate only inside the living cells of an organism. Viruses can infect all types of organisms, from animals and plants to bacteria and archaea. (

2. Any of various simple submicroscopic parasites of plants, animals, and bacteria that often cause disease and that consist essentially of a core of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms. (

3. A minute infectious agent which, with certain exceptions, is not resolved by the light microscope, lacks independent metabolism and is able to replicate only within a living host cell; the individual particle (virion) consists of nucleic acid (nucleoid)—DNA or RNA (but not both)—and a protein shell (capsid), which contains and protects the nucleic acid and which may be multilayered. (Medical Dictionary,

4. A disease caused by a virus. (


Word origin: From the Latin virus referring to poison and other noxious substances

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