Virus

A virus is “[s]elf-replicating, malicious code that attaches itself to an application program or other executable system component and leaves no obvious signs of its presence.” The program “infects” computer files, usually executable programs, by inserting a copy of itself into the file. These copies are usually executed when the infected file is loaded into memory, allowing the virus to infect other files. A virus requires human involvement (usually unwitting) to propagate.