What is a worm?

What is a worm?
A worm is computer code that spreads without user interaction. Most worms begin as email attachments that infect a computer when they're opened. The worm scans the infected computer for files, such as address books or temporary webpages, that contain email addresses. The worm uses the addresses to send infected email messages, and frequently mimics (or spoofs) the "From" addresses in later email messages so that those infected messages seem to be from someone you know. Worms then spread automatically through email messages, networks, or operating system vulnerabilities, frequently overwhelming those systems before the cause is known. Worms aren't always destructive to computers, but they usually cause computer and network performance and stability problems.