A computer virus is a program designed to replicate and spread, generally with the victim being oblivious to its existence. Computer viruses spread by attaching themselves to other programs (e.g., word processors or spreadsheets application files) or to the boot sector of a disk. When an infected file is activated - or executed - or when the computer is started from an infected disk, the virus itself is also executed. Often, it lurks in computer memory, waiting to infect the next program that is activated, or the next disk that is accessed.