Researchers say Stuxnet targeted five organizations in Iran that they believed would get them to their final target. The five organizations were believed to be the first that were infected with the worm. The worm spread from these organizations into other organizations on its way to its final target, which is believed to have been a nuclear enrichment facility or facilities in Iran. Researchers say the worm is not designed to spread via the internet but via an infected USB stick or some other targeted method within a local network. The most successful attack occurred in March 2010 when 69 percent of these infections occurred.
Source: https://thehackernews.com/2011/02/stuxnet-hit-5-gateway-targets-on-its.html