Virus, infectious agent found in virtually all life forms, including humans, animals, plants, fungi, and bacteria. Viruses consist of genetic material— DNA or RNA —surrounded by a protective coating of protein, called a capsid, with or without an outer lipid envelope. Viruses are between 20-100 times smaller than bacteria (too small to be seen by light microscopy). Viruses vary in size from the largest poxviruses of about 450 nanometers in length to the smallest polioviruses of about 30 nanometers. Viruses are not really free-living as they cannot reproduce outside of a living cell; they have evolved to transmit their genetic information from one cell to another for the purpose of replication.
Viruses can damage or kill the cells that they infect, causing disease in infected organisms. A few stimulate cells to grow uncontrollably and produce cancers. Many infectious diseases that are caused by viruses have no cures. The difficulty in developing antiviral therapies stems from the large number of variant viruses that can cause the same disease, as well as the inability of drugs to disable a virus without disabling healthy cells.
Structure and Classification Individual viruses, or virus particles, also called virions, contain genetic material in one of several form. Like cell DNA, almost all viral DNA is double-stranded, and it can have either a circular or a linear arrangement. Almost all viral RNA is single-stranded; it is usually linear, and it may be either segmented (with different genes on different RNA molecules) or nonsegmented (with all genes on a single piece of RNA).
The viral protective shell, or capsid, can be either helical or icosahedral (20 triangular sides). Capsids are composed of repeating units of one or a few different proteins. These units are called capsomers.
Viruses also carry genes for making proteins that are never incorporated into the virus particle and are found only in infected cells. These viral proteins are called non-structural proteins; they include factors required for the replication of the viral genome and the production of the virus particle.
Some icosahedral and helical animal viruses are enclosed in a lipid envelope acquired when the virus buds through host-cell membranes. Inserted into this envelope are glycoproteins that the viral genome directs the cell to make; these molecules bind virus particles to susceptible host cells.
The most elaborate viruses are the bacteriophages, which use bacteria as their hosts. Some bacteriophages resemble an insect with an icosahedral head attached to a tubular sheath. From the base of the sheath extend several long tail fibers that help the virus attach to the bacterium and inject its DNA to be replicated and to direct capsid production and virus particle assembly inside the cell