A VIRUS stand for Vital Information Resource Under Seige in the computer system. A computer virus is a type of malicious software program ("malware") that, when executed, replicates itself by modifying other computer programs and inserting its own code. Infected computer programs can include, as well, data files, or the "boot" sector of the hard drive. When this replication succeeds, the affected areas are then said to be "infected" with a computer virus.

It a chronological successor of worm programs. The computer virus was termed by Davis and Gantenbein (1987) as: "A Trojan horse program with the capability of auto-relocation (same as in worms) and it can attack other programs."

Thus, a computer virus can cause a malicious activity as bombs or Trojans but in addition can do something more.

Computer virus can get the better of the operating system which you work on, thereby taking control of the system which may sometimes lead to the destruction of all the data and programs on your hard disk.

Generally, a computer virus acts like a parasite. It draws on the resources of the computer to monitor its activities, but otherwise does not immediately change the functioning of the boot system.

This is done to evade early detection. If the virus has destructive effects, the reaction must be delayed somehow, because if it immediately destroys the host software, it will never be able to reproduce and spread.

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