Introduction to Computer Viruses

What is a virus?

A computer virus is a program that can make copies of itself. Most computer viruses do nothing more than this and are more of an annoyance than a danger. Some computer viruses, though, may also harm data and programs stored on a computer.

What types of viruses are there?

Program viruses infect computer programs and become active when the infected program is run.
Boot sector viruses infect diskettes and hard disks and become active when an infected disk is used to start the computer. (On a Macintosh, merely inserting an infected disk can activate a virus.)
Macro viruses infect documents (files) through the macro programming capabilities of some newer programs. Macro viruses become active when an infected document is opened, and the program opening the document has its macro capabilities turned on (enabled). As of late 1997, only documents created with Microsoft Word version 6 or later (Windows and Macintosh versions), Excel (5.0 for Windows 3.x and Windows NT, and 7.0 for Win95), and Lotus Ami Pro have seen infections. However, in the future, it is likely that viruses will be created that can infect other types of documents.

What is not a virus?

Trojan horse programs are designed to do something (usually something malicious) other than their supposed purpose. Trojan horse programs are sometimes classified with viruses. However, because they don't make copies of themselves, they are not true viruses.

Some programs are designed as a joke, or prank, but are not viruses. For instance, warnings of a virus incorporated in a mail message with the phrase "Good Times," "Join the Crew," or "Penpal Greetings" in the header have, at times, been rampant. The only thing that spreads, however, with these "viruses," is the messages warning people to look out for the supposed virus in their e-mail. And, it's the person sending the message, not the computer, that causes the message to be copied and spread.

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