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.

virus, computer: see computer virus.

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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Microsoft Anti-Virus (MSAV) was an antivirus program introduced by Microsoft for its MS-DOS operating system. The program first appeared in MS-DOS version 6.0 and last appeared in version 6.22. The first version of the antivirus program was quite rudimentary, had no update facility (though a single update became available), and could only scan for about 1,000 viruses. Microsoft Anti-Virus for Windows (MWAV), included as part of the package, could be run under Windows 3.x.

History

Microsoft Anti-Virus was supplied by Central Point Software Inc. (later acquired by Symantec in 1994 and integrated into Symantec's Norton AntiVirus product) and was essentially a stripped down version of the Central Point Anti-Virus (CPAV) product which, in turn, Central Point Software Inc., had licensed from Carmel Software Engineering in Haifa, Israel. Carmel Software sold the product as Turbo Anti-Virus both domestically and abroad.

Microsoft Anti-Virus for Windows was also provided by Central Point Software. This product became noted as determining that the upgrade program of Windows 95 was detected as a computer virus, something which was embarrassing to Microsoft.

Features

Both the MS-DOS and Windows versions of the latest product had common features; the "Detect and Clean" strategy of Microsoft Anti-Virus could scan for and detect 1,234 distinct viruses. Other features included the detection of boot sector and trojan horse-type viruses which was the typical virus problem at the time.

The program also had an anti-stealth and checksum feature that could be used to detect any changes in normal files. This technology was intended to make up for the unavailability of regular update packages. Only one virus update was introduced since the original releases bringing the total protection to 2,371 viruses in 1996.

Windows XP and Vista

With the acquisition of GIANT Company Software and the release of Windows Defender (formerly Microsoft AntiSpyware), and the acquisition of RAV Antivirus from GeCAD Software, Microsoft has once again entered the antivirus market with:

• Windows Live OneCare (subscription program) for Windows XP and above
• Windows Live OneCare Safety Scanner, formerly Windows Live Safety Center (free web service) for Windows 2000 and above
• Windows Defender (free program) for XP, Server 2003, and is built into Windows Vista.

External links

• List of virus detected by MSAV