An Introduction to Computer Viruses (and Other Destructive Programs)
A virus is a computer program that executes when an infected program is executed. Therefore only executable files can be infected. On MS-DOS systems, these files usually have the extensions .EXE, .COM, .BAT or .SYS. Another class of files called overlay files can also be infected. These files often have the extension .OVL, although other extensions such as .OV1 are sometimes used.