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Definition of Viruses and Virus-Like Programs
The exact definition of what constitutes a virus is a matter of some debate amongst the experts in the field. Mostly the arguments are over nuances of where to draw the line between "strict" viruses and similar programs that can best be called virus-like. In a practical sense, these subtle distinctions aren't really very important, because whether a particular program is a "true" virus or not and by what definition doesn't generally matter much to someone who has one infecting their disk! So, it is best just to define viruses based on the generally-accepted standards of the industry, and carry on.
Here is the definition I use: a computer virus is a program that attaches to other pieces of code, so that when the user tries to run the original they also unintentionally run the virus code as well; the virus code is designed to replicate itself and "infect" other programs, possibly in a modified form, and may also exhibit other behavior as well. So, in order to be a virus, the program must have the ability to do all of the following:
Note one thing that is not on this list: a virus does not necessarily have to trash your hard drive or exhibit other malicious behavior, in order to be a virus. While many viruses do damage files and disk structures, many are just nuisances or exhibit "prank" behavior such as playing music on the PC speaker or putting funny phrases on the screen when the system is booted. However, the risk of damage from viruses is substantial. Many can cause serious data loss; sometimes the virus writer doesn't even intend some of the effects that the virus has (viruses can have bugs!) Damage can also occur from program files being altered when the virus infects them--often, it is not possible to repair the damage, even when the virus is removed.
There are many different types of viruses. In addition to the classical virus, there are other virus-like programs that are similar to viruses in terms of how they work and what they do, but differ from them in one or more respect:
Note: The acronym
"WORM" is also used as a short form for "write once, read many", a
storage technology that is used by devices such as CD-R
drives. The concepts are totally unrelated.