A computer worm is a kind of malware that is able to copy itself, and spread. Unlike a computer virus, this is done without changing files on a computer, and without affecting the boot sector of media. Worms do not need human help to reproduce. Worms spread over networks, or on removable media like USB flash drives. They almost always need a helper program that is able to access the network, or that executes when a new medium is inserted. 