A virus is a biological structure composed mainly of nucleic acid within a protein coat, ranging in size from 100 to 2000 angstroms (unit of length; 1 angstrom is equal to 10 ^-10 meters, or 0.0000000001 m); they can be seen only with an electron microscope. During the stage of their replication cycle when they are free and infectious, viruses do not carry out the usual functions of living cells, such as respiration and growth; however, when they enter a living plant, animal or bacterial cell, they make use of the host cell's chemical energy and protein- and nucleic acid-synthesizing ability to replicate themselves. After viral components are made by the infected host cell, virus particles are released; the host cell is often dissolved. Some viruses do not kill cells but transform them into a cancerous state; some cause illness and then seem to disappear, while remaining latent and later causing another, sometimes much more severe, form of disease. Viruses also cause measles, mumps, yellow fever, poliomyelitis, influenza and the common cold.

Very few viruses are food-borne or cause food-borne diseases. The most important ones are mentioned below.