Submicroscopic agents that infect plants, animals and bacteria, and are unable to reproduce outside the tissues of the host. A fully formed virus consists of nucleic acid (DNA or RNA) surrounded by a protein and lipid (fat) coat. The nucleic acid of the virus interferes with nucleic acid-synthesizing mechanism of the host cell, organizing it to produce more viral nucleic acid. Viruses cause many diseases (e.g., mosaic diseases of many cultivated plants, myxomatosis, foot and mouth disease, the common cold, influenza, measles, poliomyelitis). Many plant viruses are transmitted by insects, some by eelworms. Animal viruses are spread by contact, droplet infection or by insect vectors and some are spread by the exchange of body fluids.