A computer virus is a program that is designed to copy itself into other programs. When the other programs are run, they carry out the virus’s instructions, either instead of or in addition to their own. Since one of the primary tasks 110 computer virus programmed into a virus is to reproduce itself, a virus program can spread rapidly. Viruses are generally programmed to seek out program files that are likely to be executed in the near future, such as those used by the operating system during the startup process. The result is a copy that can in turn generate an additional copy, and so on. (A virus disguised as an innocuous program is sometimes called a Trojan, short for “Trojan horse.” A distinction is sometimes made between viruses and worms. A worm generally uses flaws in a networking system to send copies to other machines, without needing to insert code into a program.)
Appearing in the 1980s, the first computer viruses were generally spread by infecting programs on floppy disks, which were often passed between users. Today, viruses generally have instructions that enable them to gain access to network facilities (such as e-mail) to facilitate their spreading to other systems on a local network or on the Internet.