What is a Computer Virus ? - Security Term

A virus is the most common malicious software that infects other files and computers. It is a computer program that can copy itself, infect a computer and alter the way it works without the permission or knowledge of the owner. The term "virus" is also commonly but mistakenly used to refer to other types of malicious software such as worms and spyware that do not have the reproductive ability.

A computer virus, like a biological one requires a host to insert its software code for replication. It also requires a host (an executable) so that it can run and spread itself. For this reason, many viruses attach themselves to executable files that may be part of genuine and legal programs. If a user attempts to launch an infected program, the virus' code may also be executed at the same time. Viruses can be divided into two types based on their activities when they are executed. Nonresident viruses instantly search for other hosts that can be infected, infect those targets, and finally transfer control to the application program they infected. Resident viruses do not search for hosts when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself.

Viruses are usually designed to corrupt files and information and are written for a specific purpose – usually to cause a certain type of symptom or damage. Some viruses are harmless pranks while others are made to create havoc.

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. Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of viruses. 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..

However, there are a number of healing options that exist after a computer has a virus. Many users install anti-virus softwares that can detect and eliminate known viruses after the computer downloads or runs the executable. Some anti-virus programs are able to scan opened files in addition to sent and received e-mails 'on the fly' in a similar manner. This is known as "on-access scanning." Many times, however, a computer that has been infected by a computer virus programma may need to be completely reinstalled or rebooted.

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