In terms of sheer number of viruses, these are the most common kind. The simplest file viruses work by locating a type of file that they know how to infect (usually a file name ending in ".COM" or ".EXE") and overwriting part of the program they are infecting. When this program is executed, the virus code executes and infects more files. These overwriting viruses do not tend to be very successful since the overwritten program rarely continues to function correctly and the virus is almost immediately discovered. The more sophisticated file viruses modify the program so that the original instructions are saved and executed after the virus finishes.
Be aware that many file viruses (such as 4096 which is also known as Frodo) also infect overlay files as well as the more usual *.COM and *.EXE files. Overlay files have various extensions, but ".OVR" and ".OVL" are common examples.
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