DEFINITION OF VIRUSES

"A program that can infect other programs by modifying them to include
a slighty altered copy of itself.A virus can spread throughout a computer
system or network using the authorization of every user using it to
infect their programs. Every programs that gets infected can also act as
a viral infection that grows "
(Fred Cohen)

The first time the term "virus" is used by Fred Cohen in 1984 in
United States. A computer virus called "virus" because it has several
fundamental equation with the virus in medical terms (biological viruses).

Computer viruses can be interpreted as a computer normal program .But
have fundamental differences with other programs, namely
virus designed to infect other programs, change,
manipulate even destructive. There is to be noted here,
the virus will infect only if the trigger program or programs that have been
infected was executed, this is where the difference with the "worm". Posts
This worm will not be discussed because the later will divert us from
discussion of this virus.read also about virus history