The thing about the stuxnet virus is that computers weren't its primary target. It was designed to target PLCs. Similar to the computer in your car, a PLC can be programmed to control quite a number of things. They are used in pretty much every production plant, power plant/distribution system, water treatment plant, drinking water distribution system, etc. In order to infect these devices (usually they don't even have a connection to the internet) the code would have had to be downloaded locally, likely from a flash drive. IIRC, the stuxnet code was written so that it would automatically transfer to anything it was connected to, but after 3 transfers it would destroy itself. This limited its geographic transfer distance-effectively containing its spread to Iran. I personally think it was a joint US/Israel effort.