A worm is a self-replicating virus designed to spread quickly, infecting as many systems as possible. The Conficker/Downadup Trojan acted as a worm, infecting millions of Microsoft systems beginning in 2008. It spread via USB stick and storage devices. Once infected, it sought out other systems containing the Microsoft vulnerability that it was exploiting. A security consortium worked to disrupt Conficker by blocking its communication delivery algorithm to ensure that a payload would never be delivered. Conficker may have been a victim of its own success, say experts. It spread too quickly and gained too much attention to make it possible to deliver an attack payload to steal data. Investigators have never found the source of the Conficker worm.