ABSTRACT
A virus is a purposefully written computer program which consists of two parts: Self-replicating code and the payload, which produces side-effects. The side-effects of a virus are limited only by the imagination of the virus author and can range from annoyance to serious vandalism. Virus side-effects are normally the first things that users notice after being infected. Viruses can be divided into four categories according to the executable item which they infect: Parasitic viruses, Bootstrap sector viruses, Multi-partite viruses and Companion viruses. Parasitic viruse modify the contents of COM and/or EXE files. Bootstrap sector viruses modify the contents of either the master bootstrap sector or the DOS bootstrap sector, depending on the virus and type of disk, usually replacing the legitimate contents with their own version. Non-memory-resident viruses are active only when an infected application is executed. The fight against viruses involves the application of five countermeasures: preparation; prevention; detection; containment; and recovery.
