"When Stuxnet moves into a computer, it attempts to spread to every machine on that computer’s network and to find out whether any are running Siemens software. If the answer is no, Stuxnet becomes a useless, inert feature on the network. If the answer is yes, the worm checks to see whether the machine is connected to a P.L.C. or waits until it is. Then it fingerprints the P.L.C. and the physical components connected to the controller, looking for a particular kind of machinery. If Stuxnet finds the piece of machinery it is looking for, it checks to see if that component is operating under certain conditions. If it is, Stuxnet injects its own rogue code into the controller, to change the way the machinery works. And even as it sabotages its target system, it fools the machine’s digital safety system into reading as if everything were normal."

Everyone should read Vanity Fair’s fantastic piece on the Stuxnet computer virus.