Worm is an independent self-replicating program spreading its copies via Internet or LAN. Traditional viruses are passive and cannot propagate themselves whereas worms can. A worm uses a network to send copies of itself to other systems, or on the lower level it uses vulnerabilities of the operation system. A worm is able to carry other malicious programs, which can perform various malicious activities, e.g. to install a backdoor in an infected computer. Even without this payload a worm is able to cause severe damage when enormously increasing the Internet traffic. As a matter of Internet expansion, a worm is able to be distributed worldwide within few hours. Side effects can be the complete congestion of network, including the businesses’ LANs.