An infectious agent composed almost entirely of protein and nucleic acids (nucleoprotein). Viruses can reproduce only within living cells and are so small that they can be resolved only with an electron microscope. Since they pass through filters that retain bacteria, they are often called filterable viruses. Tobacco mosaic was the first virus to be crystallized and isolated (Dr. W. M. Stanley, 1935); it contains some 2000 protein molecules in a sequence of 158 amino acids (mw 40,000,000). Bushy stunt virus found in tomato plants has a molecular weight of 7,600,000. First synthesis of a virus was reported in 1967.Viruses differ from organisms in that they are only half alive; they lack metabolism, are unable to utilize oxygen, to synthesize macromolecules, to grow, or to die. They are parasites, relying on a living host cell. They account for many diseases, including mumps, measles, scarlet fever, smallpox, influenza, and possibly the common cold. Their shapes are similar to those of bacteria (rods, spheres, filaments). They have the ability to mutate; they are also antigenic and thus initiate formation of antibodies. Some act as bacteriophages. A direct relation between virus and cancer has been shown, the DNA of the virus becoming irreversibly bound to the DNA of the affected cells.See Bacteria; Deoxyribonucleic Acid.
