virus
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Virus
Any of a heterogeneous class of agents that share three characteristics: (1) They consist of a nucleic acid genome surrounded by a protective protein shell, which may itself be enclosed within an envelope that includes a membrane; (2) they multiply only inside living cells, and are absolutely dependent on the host cells' synthetic and energy-yielding apparatus; (3) the initial step in multiplication is the physical separation of the viral genome from its protective shell, a process known as uncoating, which differentiates viruses from all other obligatorily intracellular parasites. In essence, viruses are nucleic acid molecules, that is, genomes that can enter cells, replicate in them, and encode proteins capable of forming protective shells around them. Terms such as “organism” and “living” are not applicable to viruses. It is preferable to refer to them as functionally active or inactive rather than living or dead.
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The primary significance of viruses lies in two areas. First, viruses destroy or modify the cells in which they multiply; they are potential pathogens capable of causing disease. Many of the most important diseases that afflict humankind, including rabies, smallpox, poliomyelitis, hepatitis, influenza, the common cold, measles, mumps, chickenpox, herpes, rubella, hemorrhagic fevers, and the acquired immunodeficiency syndrome (AIDS) are caused by viruses. Viruses also cause diseases in livestock and plants that are of great economic importance. See Acquired immune deficiency syndrome (AIDS), Plant pathology
Second, viruses provide the simplest model systems for many basic problems in biology. Their genomes are often no more than one-millionth the size of, for example, the human genome; yet the principles that govern the behavior of viral genes are the same as those that control the behavior of human genes. Viruses thus afford unrivaled opportunities for studying mechanisms that control the replication and expression of genetic material. See Human Genome Project
Although viruses differ widely in shape and size (see illustration), they are constructed according to certain common principles. Basically, viruses consist of nucleic acid and protein. The nucleic acid is the genome which contains the information necessary for virus multiplication and survival, the protein is arranged around the genome in the form of a layer or shell that is termed the capsid, and the structure consisting of shell plus nucleic acid is the nucleocapsid. Some viruses are naked nucleocapsids. In others, the nucleocapsid is surrounded by a lipid bilayer to the outside of which “spikes” composed of glycoproteins are attached; this is termed the envelope. The complete virus particle is known as the virion, a term that denotes both intactness of structure and the property of infectiousness.
Viral genomes are astonishingly diverse. Some are DNA, others RNA; some are double-stranded, others single-stranded; some are linear, others circular; some have plus polarity, other minus (or negative) polarity; some consist of one molecule, others of several (up to 12). They range from 3000 to 280,000 base pairs if double-stranded, and from 5000 to 27,000 nucleotides if single-stranded. See Virus classification
Viral genomes encode three types of genetic information. First, they encode the structural proteins of virus particles. Second, most viruses encode enzymes capable of transcribing their genomes into messenger RNA molecules that are then translated by host-cell ribosomes, as well as nucleic acid polymerases capable of replicating their genomes; many viruses also encode nonstructural proteins with catalytic and other functions necessary for virus particle maturation and morphogenesis. Third, many viruses encode proteins that interact with components of host-cell defense mechanisms against invading infectious agents. The more successful these proteins are in neutralizing these defenses, the more virulent viruses are.
The two most commonly observed virus-cell interactions are the lytic interaction, which results in virus multiplication and lysis of the host cell; and the transforming interaction, which results in the integration of the viral genome into the host genome and the permanent transformation or alteration of the host cell with respect to morphology, growth habit, and the manner in which it interacts with other cells. Transformed animal and plant cells are also capable of multiplying; they often grow into tumors, and the viruses that cause such transformation are known as tumor viruses. See Retrovirus, Tumor viruses
There is little that can be done to interfere with the growth of viruses, since they multiply within cells, using the cells' synthetic capabilities. The process, interruption of which has met with the most success in preventing virus multiplication, is the replication of viral genomes, which is almost always carried out by virus-encoded enzymes that do not exist in uninfected cells and are therefore excellent targets for antiviral chemotherapy. Another viral function that has been targeted is the cleavage of polyproteins, precursors of structural proteins, to their functional components by virus-encoded proteases; this strategy is being used with some success in AIDS patients. See Cytomegalovirus infection, Herpes, Influenza
Antiviral agents on which much interest is focused are the interferons. Interferons are cytokines or lymphokines that regulate cellular genes concerned with cell division and the functioning of the immune system. Their formation is strongly induced by virus infection; they provide the first line of defense against viral infections until antibodies begin to form. Interferons interfere with the multiplication of viruses by preventing the translation of early viral messenger RNAs. As a result, viral capsid proteins cannot be formed and no viral progeny results.
By far the most effective means of preventing viral diseases is by means of vaccines. There are two types of antiviral vaccines, inactivated virus vaccines and attenuated active virus vaccines. Most of the antiviral vaccines currently in use are of the latter kind. The principle of antiviral vaccines is that inactivated virulent or active attenuated virus particles cause the formation of antibodies that neutralize a virulent virus when it invades the body. See Animal virus, Plant viruses and viroids, Vaccination, Virus, defective
virus
[′vī·rəs]virus
(security)A virus has an "engine" - code that enables it to propagate and optionally a "payload" - what it does apart from propagating. It needs a "host" - the particular hardware and software environment on which it can run and a "trigger" - the event that starts it running.
Unlike a worm, a virus cannot infect other computers without assistance. It is propagated by vectors such as humans trading programs with their friends (see SEX). The virus may do nothing but propagate itself and then allow the program to run normally. Usually, however, after propagating silently for a while, it starts doing things like writing "cute" messages on the terminal or playing strange tricks with the display (some viruses include display hacks). Viruses written by particularly antisocial crackers may do irreversible damage, like deleting files.
By the 1990s, viruses had become a serious problem, especially among IBM PC and Macintosh users (the lack of security on these machines enables viruses to spread easily, even infecting the operating system). The production of special antivirus software has become an industry, and a number of exaggerated media reports have caused outbreaks of near hysteria among users. Many lusers tend to blame *everything* that doesn't work as they had expected on virus attacks. Accordingly, this sense of "virus" has passed into popular usage where it is often incorrectly used for a worm or Trojan horse.
See boot virus, phage. Compare back door. See also Unix conspiracy.
virus
Software used to infect a computer. After the virus code is written, it is buried within an existing program. Once that program is executed, the virus code is activated and attaches copies of itself to other programs in the computer and other computers in the network. Infected programs continue to propagate the virus, which is how it spreads.The effect of the virus may be a simple prank that pops up a message on screen out of the blue, or it may destroy programs and data right away or on a certain date. For example, the famous Michelangelo virus contaminated the machine on Michelangelo's birthday.
Viruses Must Be Run to Do Damage
A virus is a self-contained program that attaches itself to an existing application in a manner that causes it to be executed when the application is run. Macro viruses are similar. The virus code has replaced some or all of the macro commands. Likewise, it is in the execution of the macro that the damage is done (see macro language).
"In the Wild"
The term "computer virus" was coined in the early 1980s, supposedly after a graduate student presented the concept of a program that could "infect" other programs. Since then, more than a million viruses have been defined. However, the bulk of the infections are from only a few hundred active variants, said to be "in the wild."
Since 1993, the WildList Organization has been keeping track of virus attacks around the world. For more information, visit www.wildlist.org. For a sampling of different virus infections, see virus examples. See in the wild, dangerous extensions, quarantine, disinfect, macro virus, email virus, behavior detection, polymorphic virus, stealth virus, worm, boot virus, vandal, virus hoaxes and crypto rage.
Virus Theory
John von Neumann theorized that a computer program could replicate itself in his 1949 paper "Theory and Organization of Complicated Automata," and computer scientist Fred Cohen described the logic for several types of viruses in his 1984 paper "Computer Viruses - Theory and Experiments." See von Neumann architecture.
Windows vs. Mac
Almost all Windows users install an antivirus program in their computers, while many Mac users do not. Windows computers are attacked constantly, because they make up the huge majority of personal computers and are therefore the low-hanging fruit. In addition, the Mac is a Unix-based machine, and the Unix architecture separates the operating system from the applications, which makes it harder to crack, although not impossible. While the majority of Mac users do not use antivirus software, there have indeed been successful virus attacks against Macs, and Mac users are installing antivirus more than they have in the past. See antivirus program.
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| A Disease - Really? |
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| The concept of a computer "disease" seemed rather foreign in 1989 when this caption from the definition for virus in "The Computer Glossary" was published. Back then, nobody would have believed that millions of viruses were to follow. |