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companion virusA computer virus in the DOS/Windows environment that takes advantage of the fact that .COM and .EXE are two valid executable extensions for program files, and .COM takes precedence. For example, if CHKDISK were typed in at the command line, a bogus CHKDISK.COM file would be executed before the legitimate CHKDISK.EXE. In many cases, the companion virus also runs the legitimate .EXE so that the system appears normal. See virus.