virus, computer: see computer virus.

virus, parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Viruses usually 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.

Viral Structure

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. 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).

Viral Diseases

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 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 a protein, called interferon, that inhibits viral growth within the infected cell.

Classification

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.

slow virus, technically a virus, such as a lentivirus, that causes symptoms in an infected host long after the original infection and progresses slowly. Although many viruses fit this description, the term slow virus is usually reserved for the first recognized lentiviruses, such as the virus that causes visna (a disease of sheep). A slow virus was proposed as a cause for those diseases now generally recognized as prion diseases (e.g., Creutzfeld-Jacob disease and scrapie).

respiratory syncytial virus: see cold, common.

computer virus, rogue computer program, typically a short program designed to disperse copies of itself to other computers and disrupt those computers' normal operations. A computer virus usually attaches or inserts itself to or in an executable file or the boot sector (the area that contains the first instructions executed by a computer when it is started or restarted) of a disk; those that infect both files and boot records are called bimodal viruses. Although some viruses are merely disruptive, others can destroy or corrupt data or cause an operating system or applications program to malfunction. Computer viruses are spread via floppy disks, networks, or on-line services. Several thousand computer viruses are known, and on average three to five new strains are discovered every day. Virus programs can also infect advanced cellular telephones.

Antivirus programs and hardware have been developed to combat viruses. These search for evidence of a virus program (by checking for appearances or behavior that are characteristic of computer viruses), isolate infected files, and remove viruses from a computer's software. Researchers are working to sidestep the tedious process of manually analyzing viruses and creating protections against each by developing an automated immune system for computers patterned after biological processes. In 1995 Israel became the first country to legislate penalties both for those who write virus programs and those who spread the programs.

A distinction should be made between a virus—which must attach itself of another program to be transmitted—and a bomb, a worm, and a Trojan horse. A bomb is a program that resides silently in a computer's memory until it is triggered by a specific condition, such as a date. A worm is a destructive program that propagates itself over a network, reproducing as it goes. A Trojan horse is a malicious program that passes itself off as a benign application; it cannot reproduce itself and, like a virus, must be distributed by diskette or electronic mail.

West Nile virus, microorganism and the infection resulting from it, which typically produces no symptoms or a flulike condition. The virus is a flavivirus and is related to a number of viruses that cause encephalitis. It usually is transmitted through the bite of several mosquito species, and can infect humans and more than 200 animal species, including alligators, horses, and many common birds. A number of North American bird species, including the blue jay, crow, and house sparrow, act as reservoirs of the virus.

About one fifth of humans infected with the virus develop West Nile fever, which in most people is characterized by fever, headache, muscle ache, joint pain, nausea and vomiting, and in some cases rash and swollen lymph nodes (swollen glands). Less than 1% of all persons infected may develop a severe case, progressing to encephalitis or meningitis, about a week after the initial symptoms of West Nile fever appear. Loss of vision, tremors and convulsions, paralysis, coma, and, in some cases, death may result. Older persons and persons with a weakened immune system are most susceptible to severe cases of the disease, and they may suffer from longer-term effects including weakness and fatigue, headaches, memory loss and confusion, and depression. There is no treatment or vaccine for the virus except to alleviate the symptoms; controlling the mosquitoes that carry the disease is the most effective way to limit its spread.

West Nile virus was first identified in 1937 in the West Nile district of Uganda. It was subsequently found in much of the rest of Africa, the Middle East, and warmer regions of Asia and Europe. Its first recorded appearance in the United States was in Queens, N.Y., in 1999, and it since has spread to most of the United States and neighboring areas in North America.

Marburg virus: see hemorrhagic fever.

Epstein-Barr virus (EBV), herpesvirus that is the major cause of infectious mononucleosis and is associated with a number of cancers, particularly lymphomas in immunosuppressed persons, including persons with AIDS. Epstein-Barr is a ubiquitous virus, so common that it has been difficult to determine whether it is the cause of certain diseases or whether it is simply there as an artifact. In Third World nations, most children are infected with EBV; in most industrialized nations, about 50% of the people are infected. Research has found that all of the lymphomas associated with AIDS and most lymphomas in other immunocompromised persons are connected with latent EBV infection. EBV has been found in biopsy tissue of patients with Hodgkin's disease, breast cancer, and some smooth muscle tumors. EBV also was formerly suspected as the cause of chronic fatigue syndrome (originally named chronic EBV syndrome).

Ebola virus, a member of a family (Filovirus) of viruses that cause hemorrhagic fevers. The virus, named for the region in Congo (Kinshasa) where it was first identified in 1976, emerged from the rain forest, where it survives in as yet unconfirmed hosts, possibly several species of fruit bats. The virus can be fatal to chimpanzees and gorillas as well as humans.

Several strains of the virus found in Africa cause hemorrhagic fever; one found in the W Pacific does not. Once a person is infected with the virus, the disease has an incubation period of 2-21 days; however, some infected persons are asymptomatic. Initial symptoms are sudden malaise, headache, and muscle pain, progressing to high fever, vomiting, severe hemorrhaging (internally and out of the eyes and mouth) and in 50%-90% of patients, death, usually within days. The likelihood of death is governed by the virulence of the particular Ebola strain involved. Ebola virus is transmitted in body fluids and secretions; there is no evidence of transmission by casual contact. There is no vaccine and no cure.

Outbreaks of Ebola virus in humans occurred in both Congo-Kinshasa (then Zaïre) and Sudan in 1976 and 1979; other outbreaks have occurred since in Gabon, Uganda, and both Congos. Outbreaks have been exacerbated by underequipped hospitals that reused syringes and lacked proper protective clothing for personnel. In 1989 a similar virus was found in monkeys imported to the United States.

Computer program designed to copy itself into other programs, with the intention of causing mischief or damage. A virus will usually execute when it is loaded into a computer's memory. On execution, it instructs its host program to copy the viral code into any number of other programs and files stored in the computer. The corrupted programs may continue to perform their intended functions while also executing the virus's instructions, thus further propagating it. The infection may transfer itself to other computers through storage devices, computer networks, and on-line systems. A harmless virus may simply cause a cryptic message to appear when the computer is turned on; a more damaging virus can destroy valuable data. Antivirus software may be used to detect and remove viruses from a computer, but the software must be updated frequently for protection against new viruses.

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Microscopic, simple infectious agent that can multiply only in living cells of animals, plants, or bacteria. Viruses are much smaller than bacteria and consist of a single- or double-stranded nucleic acid (DNA or RNA) surrounded by a protein shell called a capsid; some viruses also have an outer envelope composed of lipids and proteins. They vary in shape. The two main classes are RNA viruses (see retrovirus) and DNA viruses. Outside of a living cell, a virus is an inactive particle, but within an appropriate host cell it becomes active, capable of taking over the cell's metabolic machinery for the production of new virus particles (virions). Some animal viruses produce latent infections, in which the virus persists in a quiet state, becoming periodically active in acute episodes, as in the case of the herpes simplex virus. An animal can respond to a viral infection in various ways, including fever, secretion of interferon, and attack by the immune system. Many human diseases, including influenza, the common cold, and AIDS, as well as many economically important plant and animal diseases, are caused by viruses. Successful vaccines have been developed to combat such viral diseases as measles, mumps, poliomyelitis, smallpox, and rubella. Drug therapy is generally not useful in controlling established viral infections, since drugs that inhibit viral development also inhibit the functions of the host cell. Seealso adenovirus; arbovirus; bacteriophage; picornavirus; plant virus; poxvirus.

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Minute infectious agent normally present in extremely small amounts in wild mice without causing obvious ill effects. It may induce cancerous tumours if grown in tissue culture and injected in large quantities into newborn mice or young hamsters, guinea pigs, and rabbits. It belongs to the Papovaviridae family of viruses.

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Any of various viruses that can cause plant disease (e.g., the tobacco mosaic virus). Plant viruses are economically important because many of them infect crop and ornamental plants. Numerous plant viruses are rodlike and can be extracted readily from plant tissue and crystallized. Most lack the fatty membrane found in many animal viruses, and all contain RNA. Plant viruses are transmitted in various ways, most importantly through insect bites, mainly by aphids and plant hoppers. Symptoms of virus infection include colour changes, dwarfing, and tissue distortion. The appearance of streaks of colour in certain tulips is caused by a virus. Seealso reovirus.

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Any of a group of viruses that cause warts and other harmless tumours in humans. More than 100 distinct types are known. Different types are responsible for warts of the hands, plantar warts (of the feet), and throat warts. Genital warts are caused by other types, which are spread by sexual intercourse. Some types of papillomaviruses that cause genital infections have been linked with various cancerous tumours, especially cervical cancers; their presence can be detected through a Pap smear.

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Virus of the Herpesviridae family that is the major cause of acute infectious mononucleosis. The virus, named for two of its discoverers, infects only salivary gland cells and one type of white blood cell. Saliva is the only bodily fluid that has been proved to contain infectious EBV particles. In less-developed nations, infection with EBV occurs in almost all children before the age of 5 and is not associated with recognizable symptoms. When EBV infection is delayed until the teen or early adult years, the body commonly responds differently, resulting in mononucleosis. Other, rarer disorders have also been linked with EBV, including certain cancers. There are no specific treatments for any form of EBV infection, and no vaccines have been developed.

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