virus, parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes. In one stage of their life cycle, in which they are free and infectious, virus particles do not carry out the functions of living cells, such as respiration and growth; in the other stage, however, viruses enter living plant, animal, or bacterial cells and make use of the host cell's chemical energy and its protein- and nucleic acid–synthesizing ability to replicate themselves.
The existence of submicroscopic infectious agents was suspected by the end of the 19th cent.; in 1892 the Russian botanist Dimitri Iwanowski showed that the sap from tobacco plants infected with mosaic disease, even after being passed through a porcelain filter known to retain all bacteria, contained an agent that could infect other tobacco plants. In 1900 a similarly filterable agent was reported for foot-and-mouth disease of cattle. In 1935 the American virologist W. M. Stanley crystallized tobacco mosaic virus; for that work Stanley shared the 1946 Nobel Prize in Chemistry with J. H. Northrup and J. B. Summer. Later studies of virus crystals established that the crystals were composed of individual virus particles, or virions. By the early 21st cent. the understanding of viruses had grown to the point where scientists synthesized (2002) a strain of poliovirus using their knowledge of that virus's genetic code and chemical components required.
Typically the protein coat, or capsid, of an individual virus particle, or virion, is composed of multiple copies of one or several types of protein subunits, or capsomeres. Some viruses contain enzymes, and some have an outer membranous envelope. Many viruses have striking geometrically regular shapes, with helical structure as in tobacco mosaic virus, polyhedral (often icosahedral) symmetry as in herpes virus, or more complex mixtures of arrangements as in large viruses, such as the pox viruses and the larger bacterial viruses, or bacteriophages. Certain viruses, such as bacteriophages, have complex protein tails. The inner viral genetic material—the nucleic acid—may be double stranded, with two complementary strands, or single stranded; it may be deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The nucleic acid specifies information for the synthesis of from a few to 50 different proteins, depending on the type of virus.
Viral Infection of a Host Cell
A free virus particle may be thought of as a packaging device by which viral genetic material can be introduced into appropriate host cells, which the virus can recognize by means of proteins on its outermost surface. A bacterial virus infects the cell by attaching fibers of its protein tail to a specific receptor site on the bacterial cell wall and then injecting the nucleic acid into the host, leaving the empty capsid outside. In viruses with a membrane envelope the nucleocapsid (capsid plus nucleic acid) enters the cell cytoplasm by a process in which the viral envelope merges with a host cell membrane, often the membrane delimiting an endocytic structure (see endocytosis) in which the virus has been engulfed.
Within the cell the virus nucleic acid uses the host machinery to make copies of the viral nucleic acid as well as enzymes needed by the virus and coats and enveloping proteins, the coat proteins of the virus. The details of the process by which the information in viral nucleic acid is expressed and the sites in the cell where the virus locates vary according to the type of nucleic acid the virus contains and other viral features. As viral components are formed within a host cell, virions are created by a self-assembly process; that is, capsomere subunits spontaneously assemble into a protein coat around the nucleic core. Release of virus particles from the host may occur by lysis of the host cell, as in bacteria, or by budding from the host cell's surface that provides the envelope of membrane-enveloped forms.
Some viruses do not kill host cells but rather persist within them in one form or another. For example, certain of the viruses that can transform cells into a cancerous state (see cancer) are retroviruses; their genetic material is RNA but they carry an enzyme that can copy the RNA's information into DNA molecules, which then can integrate into the genetic apparatus of the host cell and reside there, generating corresponding products via host cell machinery (see also retrovirus). Similarly, in bacterial DNA viruses known as temperate phages, the viral nucleic acid becomes integrated into the host cell chromosomal material, a condition known as lysogeny; lysogenic phages are similar in many ways to genetic particles in bacterial cells called episomes (see recombination).
Some human diseases are apparently caused by the body's response to virus infection: immune reaction to altered virus-infected cells, release by infected cells of inflammatory substances, or circulation in the body of virus-antibody complexes are all virus-caused immunological disorders. Viruses cause many diseases of economically important animals and plants, some transmitted by carriers such as insects. A retrovirus (HIV) causes AIDS, several viruses (e.g. Epstein-Barr virus, human papillomavirus) cause particular forms of cancer in humans, and many have been shown to cause tumors in animals. Other viruses that infect humans cause measles, mumps, smallpox, yellow fever, rabies, poliomyelitis, influenza, and the common cold.
The techniques of molecular biology and genetic engineering have made possible the development of antiviral drugs effective against a variety of viral infections. Viruses, like bacterial infective agents, act as antigens in the body and elicit the formation of antibodies in an infected individual (see immunity). Indeed, vaccines against viral diseases such as smallpox were developed before the causative agents were known. Some viruses stimulate cellular production of interferon, which inhibits viral growth within the infected cell.
Viruses are not usually classified into conventional taxonomic groups but are usually grouped according to such properties as size, the type of nucleic acid they contain, the structure of the capsid and the number of protein subunits in it, host species, and immunological characteristics.
See C. Zimmer, A Planet of Viruses (2011).