A worm is a self-propagating program that can automatically distribute itself from one computer to another. Worms may propagate themselves using one or more of the following methods:

• Email programs: A worm may propagate via email messages by circulating as an attachment or via a link in the message. In some instances the worm creates and sends out email messages containing a copy of itself as an attachment. In other cases, the worm creates and sends out email messages containing a link to a certain website; the link then leads to a copy of itself.
• Instant messaging programs: A worm may spread through instant messaging (IM) applications, such as Windows Live Messenger and AOL Instant Messenger, typically by sending IM messages that include a link to a copy of itself.
• File-sharing programs: A worm may propagate via file-sharing or peer-to-peer programs. It usually creates copies of itself in common download/upload folders of these programs, with file names taken from popular software or games. This social engineering technique makes it more likely that a user who wishes to download a counterfeit version of such popular software or games instead downloads and eventually runs a copy of the worm.
• Social networking sites: A worm may spread by automatically sending messages to all of a user's contacts in a social networking website, such as Facebook and MySpace. The message usually contains a link to a copy of itself.
• Network shares: A worm may propagate via network shares and mapped drives. These types of worms can spread by creating copies of itself in shared folders. If these folders are password-protected, some worms may attempt to access the share by using commonly used user names and passwords.
• Removable drives with Autorun enabled: A worm may propagate via removable drives such as flash drives and portable hard disks. These types of worms are called autorun worms because, aside from creating copies of itself in removable drives, they also drop a file, usually called autorun.inf. This INF file enables the worm copy to automatically run when the drive is accessed and Autorun is enabled. Autorun is the same functionality that allows, for example, your CD drive to automatically run a setup file when you install software, or play music when you insert a music CD.
• Software vulnerabilities: A worm may exploit a wormable vulnerability in certain software to spread to other computers. Examples of exploit worms are the Sasser and Blaster outbreaks, and the Confickerworm that first appeared in 2008. These worms exploited vulnerabilities in Windows services that are commonly used by computers to communicate with each other; hence a clean computer that communicates with an infected computer runs the risk of being infected itself.