A computer virus is a computer program that is designed to replicate itself from file to file or from disk to disk. Viruses comprise the same basic instructions and program logic that constitute application programs, such as word processors, games, or spreadsheets. However, the computer virus is designed to spread itself without the user's knowledge, while normal applications are designed to provide a useful function. Computer viruses are written by people, and they must be intentionally designed and programmed to self-replicate. However, once a computer virus has been introduced into a system, it is capable of spreading without the aid of a human.