Spread over 200 countries and tens of millions of computers and servers, Conficker is the most virulant worm of all time. The worm combines several malware techniques and is also part virus and part trojan. It attempts to block system updates and attacks anti-malware programs. Conficker has been particularly devastating in Europe, striking the UK ministry of defense, the French Navy and Norwegian Police. Several variants of the worm were created in order to keep it one step ahead of attempts to wipe it out. It remains the most ruthlessly effective malware program of the modern era.