A
computer virus
is a
computer program
that can copy itself
and infect a
computer without the
permission or
knowledge of the
user. The term
"virus" is also
commonly but
erroneously used to
refer to other types
of
malware,
adware and
spyware programs
that do not have the
reproductive
ability. A true
virus can only
spread from one
computer to another
(in some form of
executable
code) when its
host is taken to the
target computer; for
instance because a
user sent it over a
network or the
Internet, or carried
it on a removable
medium such as a
floppy disk,
CD, or
USB drive.
Viruses can increase
their chances of
spreading to other
computers by
infecting files on a
network file system
or a file system
that is accessed by
another computer.
Viruses are
sometimes confused
with
computer worms
and
Trojan horses,
which are
technically
different. A worm
can spread itself to
other computers
without needing to
be transferred as
part of a host, and
a Trojan horse is a
program that appears
harmless but has a
hidden agenda. Worms
and Trojans, like
viruses, may cause
harm to either a
computer system's
hosted data,
functional
performance, or
networking
throughput, when
they are executed.
Some viruses and
other malware have
symptoms noticeable
to the computer
user, but most are
surreptitious. This
makes it hard for
the average user to
notice, find and
disable and is why
specialist
anti-virus programs
are now commonplace.
Most personal
computers are now
connected to the
Internet and to
local area networks,
facilitating the
spread of malicious
code. Today's
viruses may also
take advantage of
network services
such as the
World Wide Web,
e-mail,
Instant Messaging
and
file sharing
systems to spread,
blurring the line
between viruses and
worms. Furthermore,
some sources use an
alternative
terminology in which
a virus is any form
of self-replicating
malware.
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