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What is virus Computer?

Well, the best definition we have been able to come up with is the
following

1. A virus is a program that is able to replicate, that is create
(possibly modified) copies of itself.
2. The replication is intentional, not just a side-effect.
3. At least some of replicants in turn are also viruses by the same
definition.
4. A virus has to attach itself to a "host", in the sense that
execution of the host implies execution of the virus.

virus may be modified, either by the original author or someone else, so
that a more harmful version of it appears. It is also possible that the
modification produces a less harmful virus, but that has only rarely
happened.

The damage caused by a virus may consist of the deletion of data or
programs, maybe even reformatting of the hard disk, but more subtle damage
is also possible. Some viruses may modify data or introduce typing errors
into text. Other viruses may have no intentional effects other than just
replicating.
Finally, remember that some viruses may interfere with the disinfection
operation if they are active in memory at that time, so before attempting
to disinfect you MUST boot the computer from a CLEAN system diskette -
preferaply one that has been kept write-protected since it was originally
created.

It is also a good idea to boot from a clean system diskette before
scanning for viruses, as several "stealth" viruses are very difficult do
detect if they are active in memory during virus scanning.