A worm is a type of computer program that tries to spread by replicating itself from computer to computer using a network connection (usually, but not always, the Internet). It will often attack a computer by using known vulnerabilities (bugs) in the networking part of the operating system.

Worms often deliver a payload such as a trojan or backdoor, so can be very damaging to your computer. Worms also often launch denial of service attacks and other remote threats, using your computer as a “zombie” host to hide the attacker's identity.

To guard against worms, protect your computer by updating the operating system regularly (see the section called “Patching and Updating”) and by keeping your virus protection up to date.