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A type of intrusive malware that replicates itself and inserts copies of itself in legitimate programs, where it carries out unwanted and often damaging operations. Viruses initially were spread through infected floppy disks, which users frequently exchanged to share data and software. The most common contemporary methods of propagation are through attachments to Internet e-mail and programs downloaded from Websites. Viruses can be prevented if users open attachments only from trusted correspon dents, visit only trusted websites, and purchase anti-virus software that they keep current. The term virus, in the contemporary context, was first used by Fred Cohen in his paper "Experiments with Computer Viruses" (1984). According to Cohen, the term was coined by Len Adleman; however, the term was in common usage long before.The science fiction novel When HARLIE was One (1972), by David Gerrold, describes a computer program named VIRUS, which could be countered by a program named VACCINE. See also malware, spyware, Trojan horse, and worm.

See virus in Webster''s New World Hacker Dictionary

Can be a harmful, self-replicating program usually hidden in another piece of computer code, such as an email message. However, some virus infections are purely host-based, so they do their Â“black magicÂ” only locally.

Because viruses replicate across a network in a variety of ways, they can cause Denial of Service (DoS) attacks in which the victim is not specifically targeted but is an unlucky host. Depending on the type of virus, the DoS can be hardly noticeableÂ—or it can cause a major Âdisaster.

A security expert and content editor for SymantecÂ’s online magazine SecurityFocus notes that as of April 2005, Windows users had experienced more than 140,000 virus attacks, in contrast to the Macintosh Apple users who had experienced none. Some security experts maintain that AppleÂ’s freedom from viruses is caused by a lack of critical mass, but SymantecÂ’s expert thinks it is a combination of AppleÂ’s OS X operating system and its three-tiered user-privilege systemÂ—(i) user, (ii) GUI superuser, and (iii) rootÂ—that is disabled by default. Perhaps that is why, says the Symantec security professional, that Apple experiences a 70% year-over-year growth in Ârevenues.

See Also: Denial of Service (DoS); Electronic Message or Email; Root; Symantec Corporation.

Goldberg, I. Glossary of Information Warfare Terms. [Online, October 27, 2003.] Institute for the Advanced Study of Information Warfare. http://www.psycom.net/iwar.2 .html; Martin, K. AppleÂ’s Big Virus. [Online, April 21, 2005.] Reg SETI Group Website. http:// www.theregister.co.uk/2005/04/21/apples_big_virus/; TechTarget. Denial of Service. [Online, May 16, 2001.] TechTarget Website. http://searchsecurity.techtarget.com/sDefinition/0,,sid14_ gci213591,00.htm.

See virus in Computer

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 system. Infected programs copy the virus to other programs.

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. It can lay dormant and do its damage once a year. For example, the Michelangelo virus contaminates the machine on Michelangelo's birthday.

Viruses Must Be Run to Do Damage
A virus is not inserted into data. It is a self-contained program or code that attaches itself to an existing application in a manner that causes it to be executed when the application is run. Macro viruses, although hidden within documents (data), are similar. It is in the execution of the macro that the damage is done.

E-Mail Attachments Are Suspect
Files attached to e-mail messages are a common way of infecting a computer when the recipient is not aware of file types that are potentially harmful. For example, files with extensions such as .EXE, .BAT and .COM can perform any operation within the computer and should never be clicked unless the user is expecting the attachment. See dangerous extensions and double extension.

Viruses Are Relatively Recent
The term 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 80,000 viruses have been defined. However, 99% of the infections are from only a few hundred variants found "in the wild."