What is companion virus?
A companion virus is a complicated computer virus that, unlike traditional viruses, does not alter files. Instead, it makes a copy of the file and adds a different extension, usually .com. This unique quality makes it difficult to detect an accompanying virus, as antivirus software tends to use changes in files as a guide.
The companion virus is an old type of virus that became more prominent during the MS-DOS era. It is mainly spread through human intervention.
Typically, when the user runs a program using the command prompt, they enter the name of the program. Since MS-DOS does not require the file type to be specified, the first file name that corresponds to what the user enters is automatically executed. So if a companion virus copies file.exe and renames file.com because file.com comes before file.exe, MS-DOS runs the first program, spreading the infection on the computer without the user knowing about it.
Companion viruses often require human intervention to further infect a computer, and with the advent of Windows XP, which no longer makes heavy use of the MS-DOS interface, there have been fewer opportunities for this type of virus to spread. However, it can still work if a user accidentally double-clicks it or runs it accidentally, especially if the 'Show file extensions' option is not checked.