Worms

A worm is a program that actively seeks aout more machines to infect and each infected machine serves as an automated launching pad for attacks on other machines.

This approach exploits software vulnerabilities in client or server programs. They can use network connections to spread from system to system. They can spread through shared media such as USB drives, CD, etc....

Email worms sprad in macro or script code included in attachments and instant messenger file transfers.

Upon activation the worm may replicate and propagate again, and they usually carry some form of payload.