Worms are malicious programs that replicate or execute over computer networks or hard drives. The main difference between a worm and a virus is that a virus piggy-backs on other applications whereas a worm depends on computer networks to spread itself. Once installed on a system, worms may also open TCP ports to make networks security holes for other programs, and they may try to “flood” the LAN with fake Denial of Service (DoS) data transmissions.