[Phil Lelyveld comment; this is a long and fascinating story. The idea of deploying thousands of units under the assumption that only one or two have to actually complete the mission was also the idea behind the last few missions to Mars and is the idea behind a lot of field-robotics development.]
The nuclear facility in Iran runs an “air gap” security system, meaning it has no connections to the Web, making it secure from outside penetration. Stuxnet was designed and sent into the area around Iran's Natanz nuclear power plant -- just how may never be known -- to infect a number of computers on the assumption that someone working in the plant would take work home on a flash drive, acquire the worm and then bring it back to the plant.
full story here