What is a worm?

A worm is computer code that spreads without user interaction. Most worms begin as e‑mail 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 e‑mail addresses. The worm uses the addresses to send infected e‑mail, and often mimics (or spoofs) the "From" addresses in subsequent e‑mail messages so that those infected messages appear to be from someone you know. Worms then spread automatically through e‑mail, networks, or operating system vulnerabilities, often 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.