File viruses usually replace or attach themselves to COM and EXE files. They can also infect files with the extensions SYS, DRV, BIN, OVL, and OVY. File viruses may be resident or non-resident, the most common being resident or TSR (terminate-and-stay-resident) viruses. Many non-resident viruses simply infect one or more files whenever an infected file runs. These are also called parasitic viruses, file infectors, or file infecting viruses.