What is a companion virus?

A COMPANION virus is one that, instead of modifying an existing file, creates a new program which (unknown to the user) is executed instead of the intended program. On exit, the new program executes the original program so that things appear normal. On PCs this has usually been accomplished by creating an infected .COM file with the same name as an existing .EXE file. Integrity checking antivirus software that only looks for modifications in existing files will fail to detect such viruses.