Stuxnet is considered a multi-part worm, especially in the case of attacks on Iranian nuclear facilities. The computer worm first travels via USB sticks. When the time comes for spreading, Stuxnet spreads through computers running on the Microsoft Windows operating system.
Once in a system of computers, Stuxnet runs a search for infected devices with specific software on them. The Iranian attack had a Stuxnet variant that specifically targeted computers serving as programmable logic controllers to monitor and automate plant equipment.