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 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. In that way, a worm could start a program to send electronic mail, and send itself on to all the addresses in the address book of the email application. A very famous worm that appeared in 2000 was called ILOVEYOU (or Love Letter), it sent a simple mail with the subject ILOVEYOU, and an attachment of a document that contained the worm code.