A worm is a type of malware that self replicates. The user does not need to take a specific action for the malware to replicate. Worms generally spread from system to system across the network in a fast period of time.
Firewalls and IDS/IPS systems can help to stop the spread of worms into a network if it’s detected. But once the worm is inside the system, there’s not much that can be done.
