A specific type of virus where the infected code is stored not in the host program, but in a separate ‘companion’ file. For example, the virus might rename the standard NOTEPAD.EXE file to NOTEPAD.EXD and create a new NOTEPAD.EXE containing the virus code. When the user subsequently runs the Notepad application, the virus will run first and then pass control to the original program, so the user doesn’t see anything suspicious.

Read more:

• About classic viruses