Companion viruses use a feature of DOS that allows software programmes with the same name, but with different extensions, to operate with different priorities. Most companion viruses create a COM file which has a higher priority than an EXE file with the same name. Thus, a virus may see a system contains the file PROGRAM. EXE and create a file called PROGRAM. COM. When the computer executes programme from the command line, the virus (PROGRAM. COM) runs before the actual PROGRAM. EXE. Often the virus will execute the original programme afterwards so the system appears normal.