A computer programme file capable of attaching to discs 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 files. Some viruses display symptoms, others damage files and computer systems, but neither is essential in the definition of a virus; a non-damaging virus is still a virus.