VIRUS... non-living molecular particles of nucleic acid and protein coats.

     A large group of infectious agents ranging from 10 to 250 nanometers in diameter, composed of a protein sheath surrounding a nucleic acid core and capable of infecting all animals, plants, and bacteria; characterized by total dependence on living cells for reproduction and by lack of independent metabolism.

     Viruses are obligate parasites that must enter host cells to reproduce. Because of their relatively simple structures, they replicate rapidly (generally in less than a day, sometimes in 2 to 3 h) & prolifically (each infected cell produces 100-10,000 new viruses). The combination of a low fidelity of replicating their genetic information & the ability to incorporate host genes into their own genome results in an extraordinary ability to mutate.