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A computer virus is a computer program that can copy itself and infect a
computer without permission or knowledge of the user. The original may modify
the copies or the copies may modify themselves, as occurs in a metamorphic virus
. 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
carrying it on a removable medium such as a floppy disk, CD, USB drive or by the
Internet. Additionally, viruses can spread 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. A worm,
however, can spread itself to other computers without needing to be transferred as
part of a host.
A Trojan horse is a file that appears harmless until executed. In contrast
to viruses, Trojan horses do not insert their code into other computer
files. Many personal computers are now connected to the Internet and to
local-area networks, facilitating their spread.
In the context of computer software, a Trojan horse is a program that installs
malicious software while under the guise of doing something else. A Trojan horse
differs from a virus in that a Trojan horse does not insert its code into other
computer files and appears harmless until executed. The term is derived from the
classical myth of the Trojan Horse. Trojan horses may appear to be useful or
interesting programs (or at the very least harmless) to an unsuspecting user, but
are actually harmful when executed. (See Social engineering.)
Often the term is shortened to simply Trojan.
There are two common types of Trojan horses. One is ordinary software that has
been corrupted by a hacker. A hacker inserts malicious code into the program that
executes while the program is used. Examples include various implementations of
weather alerting programs, computer clock setting software, and peer-to-peer file
sharing utilities. The other type of Trojan is a standalone program that
masquerades as something else, like a game or image file, in order to trick the
user into running the program.
Trojan horse programs cannot operate autonomously, in contrast to some other
types of malware, like viruses or worms. Just as the Greeks needed the Trojans
to bring the horse inside for their plan to work, Trojan horse programs depend on
actions by the intended victims. As such, if Trojans replicate and distribute
themselves, each new victim must run the Trojan. Therefore their virulence is of a
different nature, depending on successful implementation of social engineering
concepts rather than flaws in a computer system's security design or
In the field of computer architecture, 'Trojan Horse' can also refer to security
loopholes that allow Kernel Code to access anything for which it is not authorized.
Some viruses are programmed to damage the computer by damaging
programs, deleting files, or reformatting the hard disk. Others are not
designed to do any damage, but simply replicate themselves and perhaps
make their presence known by presenting text, video, or audio messages.
Even these benign viruses can create problems for the computer user.
They typically take up computer memory used by legitimate programs. As
a result, they often cause erratic behavior and can result in system
crashes. In addition, many viruses are bug-ridden, and these bugs may
lead to system crashes and data loss.
There are many viruses operating in the general Internet today, and new
ones are created and discovered every day.
Today's viruses may also take advantage of
network services such as the World Wide Web,
e-mail, 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
The role of software development
Because software is often designed with security features to prevent
unauthorized use of system resources, many viruses must exploit
software bugs in a system or application to spread. Software development
strategies that produce large numbers of bugs will generally also produce
Anti-virus software and other preventive
There are two common methods that an anti-virus software application uses to
detect viruses. The first, and by far the most common method of virus detection
is using a list of virus signature definitions. The disadvantage of this detection
method is that users are only protected from viruses that pre-date their last virus
definition update. The second method is to use a heuristic algorithm to find
viruses based on common behaviors. This method has the ability to detect
viruses that anti-virus security firms’ have yet to create a signature for.
Many users install anti-virus software that can detect and eliminate known
viruses after the computer downloads or runs the executable. They work by
examining the content heuristics of the computer's memory (its RAM, and
boot sectors) and the files stored on fixed or removable drives (hard drives,
floppy drives), and comparing those files against a database of known virus
"signatures". Some anti-virus programs are able to scan opened files in addition
to sent and received emails 'on the fly' in a similar manner. This practice is
known as "on-access scanning." Anti-virus software does not change the
underlying capability of host software to transmit viruses. Users must update
their software regularly to patch security holes. Anti-virus software also needs to
be regularly updated in order to gain knowledge about the latest threats.
Once a computer has been compromised by a virus, it is usually unsafe to
continue using the same computer without completely reinstalling the
operating system. However, there are a number of recovery options that
exist after a computer has a virus. These actions depend on severity of the
type of virus.
opeRating system Reinstallation
As a last ditch effort, if a virus is on your system and anti-viral software can't
clean it, then reinstalling the operating system may be required. To do this
properly, the hard drive is completely erased (partition deleted and formatted)
and the operating system is installed from media known not to be infected.
Important files should first be backed up, if possible, and separately scanned
for infection before erasing the original hard drive and reinstalling the
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