A virus is a computer program file capable of attaching to disks or other files and replicating itself repeatedly, typically without user knowledge or permission. Some viruses attach to files so when the infected file executes, the virus also executes. Other viruses sit in a computer's memory and infect files as the computer opens, modifies, or creates the files. Some viruses display symptoms, and others damage files and computer systems, but neither is essential in the definition of a virus; a non-damaging virus is still a virus. There are computer viruses written for several operating systems including DOS, Windows, Amiga, Macintosh, Atari, UNIX, and others. McAfee. Com presently detects more than 57,000 viruses, Trojans, and other malicious software. Also see: boot sector infector, file viruses, macro virus, companion virus, worm.