A
computer virus is a computer program that can copy itself and infect a
computer. 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, DVD, 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.
As stated above, the term "computer virus" is
sometimes used as a catch-all phrase to include all types of malware,
adware, and spyware programs that do not have the reproductive ability.
Malware includes computer viruses, worms, trojans, most rootkits,
spyware, dishonest adware, crimeware, and other malicious and unwanted
software, including true viruses. Viruses are sometimes confused with
computer worms and Trojan horses, which are technically different. A
worm can exploit security vulnerabilities to spread itself
automatically to other computers through networks, while a Trojan is a
program that appears harmless but hides malicious functions. Worms and
Trojans, like viruses, may harm a computer system's data or
performance. Some viruses and other malware have symptoms noticeable to
the computer user, but many are surreptitious and go unnoticed.
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