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Virus Overview

Definition
The term "computer virus" generically refers to one of three different types of programs:

Virus: A program that infects files but relies on humans to be copied from computer to computer.
Trojan horse: A program that contains a virus "payload", which infects the computer when the program is run.
Trojan horses cannot reproduce and rely on humans to spread themselves.
Worm: A program that attempts to spread itself from computer to computer, usually without human help.

Viruses are either malignant (cause data damage or loss) or benign (cause little or no damage).

Myths (from Symantec Corp.; www.symantec.com)
While viruses are capable of damaging systems, they cannot do the following:

Viruses don't infect files on write-protected disks.
Viruses don't infect compressed files. However, applications within a compressed file could have been infected before they were compressed. Some viruses are known to insert copies of themselves in already-created archives.
Viruses don't infect computer hardware such as monitors or computer chips; they only infect software. They can, however, damage certain types of hardware such as flash-memory.
Macintosh viruses don't infect DOS-based computer software, and vice versa. For example, the Michelangelo virus does not infect Macintosh applications. Again, an exception to this rule are the Word and Excel macro viruses, which infect spreadsheets, documents, and templates which can be opened by either Windows or Macintosh computers.
Viruses usually do not identify themselves as viruses, even after they do something destructive.

Virus Hoaxes
A virus hoax is usually an e-mail message that warns the user of a virus that will cause data loss. In the case of a virus hoax, however, no virus (as described in the e-mail message) exists.

For more information on virus hoaxes, see our Virus Hoaxes page.

Further Reading
Symantec Corporation's Virus Descriptions



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