The term "virus" is often used generically to identify any harmful migrating computer program. However, more strictly defined, a "worm" is a program that travels from one computer to another, usually over a network, but does not attach itself to the operating system of the computer it "infects." It replicates itself until the host computer runs out of memory or disk space. A "Trojan horse" is a piece of computer software that acts like it has a benign purpose, but is actually performing an ulterior malicious command, such as erasing files. A "virus" insidiously attaches itself to the operating system of any computer it enters and can infect any other computer that uses files from the infected computer.