The Conficker worm was first released in November 2008 and later followed up by increasingly sophisticated variants. Targeting a vulnerability in the Microsoft Windows operating system, the worm can spread by means of networks and USB flash drives. Millions of computers were rapidly infected; this map from security firm Team Cymru shows a snapshot of detected infections in January 2009. Since then, Conficker has proved difficult to eradicate. Its resilience is due in part to its ability to disable antivirus software, block users from accessing the websites of security vendors to get new software, and download encrypted updates designed to defeat counter­measures against previous versions of Conficker. Microsoft is offering a $250,000 reward for the identities of the worm's creators.