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.
By definition, a virus infects other programs with
copies of itself. It has the ability to clone itself, so that it can
multiply, constantly seeking new host environments. The
most harmless viruses do only that, simply replicating and
spreading to new systems. Or the virus program may
damage other programs and/or alter data, perhaps selfdestructing
when done. The only evidence viruses like this
leave is the destruction they have inflicted on the infected
system. This makes it very difficult to develop defenses
against the virus.