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A companion virus is a complicated computer virus which, unlike traditional viruses, does not modify any files. Instead, it creates a copy of the file and places a different extension on it, usually .com. This unique quality makes a companion virus difficult to detect, as anti-virus software tends to use changes in files as clue.
The companion virus is an old type of virus that was more prominent during the MS-DOS era. It is propagated mostly through human intervention.
When the user executes a program with the command prompt, he or she usually types in the name of the program. Because MS-DOS does not need a specification of the file type, it automatically runs the first file name that matches what the user types. So, if a companion virus copies file.exe and renames it file.com, because file.com comes before file.exe, MS-DOS will run that first program, thus spreading the infection in the computer, unbeknownst to the user.
Companion viruses mostly need human intervention to further infect a computer and with the advent of Windows XP, which does not use the MS-DOS interface much anymore, there were fewer ways for this type of virus to propagate itself. However, it can still work if a user double clicks it unintentionally or is run by accident, especially if the “show file extensions” option is not activated.