/ˈvaɪrəs/
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. (wikipedia.com)
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. (TheFreeDictionary.com)
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, TheFreeDictionary.com)
4. A disease caused by a virus. (TheFreeDictionary.com)
Word origin: From the Latin virus referring to poison and other noxious substances