Definition of a Computer Virus
A computer virus is a program designed to spread itself by first
infecting executable files or the system areas of hard and floppy
disks and then making copies of itself. Viruses usually operate
without the knowledge or desire of the computer user, often causing
harm to the users computer system.
Viruses have the potential to infect any type of executable code,
not just the files that are commonly called 'program files'. For
example, some viruses infect executable code in the boot sector
of floppy disks or in system areas of hard drives. Another type
of virus, known as a 'macro' virus, can infect word processing and
spreadsheet documents that use macros. And it's possible for HTML
to spread viruses or other malicious code.
Since virus code must be executed to have any effect, files that
the computer treats as pure data are safe. This includes graphics
and sound files such as .gif, .jpg, .mp3, .wav, etc., as well as
plain text in .txt files. For example, just viewing picture files
won't infect your computer with a virus. The virus code has to be
in a form, such as an .exe program file or a Word .doc file, that
the computer will actually try to execute.
When you execute program code that's infected by a virus, the
virus code will also run and try to infect other programs, either
on the same computer or on other computers connected to it over
a network. And the newly infected programs will try to infect yet
When you share a copy of an infected file with other computer
users, running the file may also infect their computers; and files
from those computers may spread the infection to yet more computers.
If your computer is infected with a boot sector virus, the virus
tries to write copies of itself to the system areas of floppy disks
and hard disks. Then the infected floppy disks may infect other
computers that boot from them, and the virus copy on the hard disk
will try to infect still more floppies.
Some viruses, known as 'multipartite' viruses, can spread both
by infecting files and by infecting the boot areas of floppy disks.