A computer virus is a program that can copy itself without permission and infect other computers. It spreads by attaching itself to other programs and files that are transferred between computers. Viruses replicate by piggybacking on real programs and files and automatically spreading to other computers through email attachments or by exploiting security vulnerabilities over networks. The first computer virus was created in 1986 and was named "Brain".

2. A computer virus is a computer
program that can copy itself and
infect a computer without permission
or knowledge of the user. The term
"virus" is also commonly used, albeit
erroneously, to refer to many different
types of malware and adware
programs. The original virus may
modify the copies, or the copies may
modify themselves, as occurs in a
metamorphic virus.

3.  A virus can only spread from one computer to
another when its host is taken to the
uninfected computer, for instance by a user
sending it over a network or the Internet, or
by carrying it on a removable medium such
as a floppy disk, CD, or USB drive.

5.  A virus is a small piece of software that
piggybacks on real programs. For example, a
virus might attach itself to a program such as
a spreadsheet program. Each time the
spreadsheet program runs, the virus runs,
too, and it has the chance to reproduce (by
attaching to other programs)

6.  An e-mail virus travels as an attachment to
e-mail messages, and usually replicates itself
by automatically mailing itself to dozens of
people in the victim's e-mail address book.
Some e-mail viruses don't even require a
double-click -- they launch when you view
the infected message in the preview pane of
your e-mail software

7.  A Trojan horse is simply a computer program.
The program claims to do one thing (it may
claim to be a game) but instead does damage
when you run it (it may erase your hard disk).
Trojan horses have no way to replicate
automatically.

8.  A worm is a small piece of software that uses
computer networks and security holes to
replicate itself. A copy of the worm scans the
network for another machine that has a
specific security hole. It copies itself to the
new machine using the security hole, and
then starts replicating from there, as well.

9. The first ever computer virus was developed in
1986 by two Pakistani brothers in Lahore
Pakistan.
It was named “Brain” and was designed by
Amjad Farooq Alvi and Basit Farooq Alvi with
the intention of discouraging the piracy of a
software written by them.