Program code written to replicate by attaching copies of itself to other objects within the system, and normally also having a detrimental effect. This may range from generation of irritating messages, through denial of service, to corruption or complete destruction of data. A program virus will seek out and copy itself into other program files whenever a previously infected program is run. A boot sector virus copies itself into that sector of a disk and spreads whenever a system is boot loaded from an infected disk. Viruses are spread when infected programs or disks are transferred to previously clean systems. See also logic bomb, Trojan horse, worm.