A new book asserts that the Stuxnet worm that attacked Iran's Natanz uranium enrichment facility entered the plant by first infecting four companies that were performing contracting work there.  The book, "Countdown to Zero Day: Stuxnet and the Launch of the World's First Digital Weapon," was written by Wired reporter Kim Zetter.  Zetter claims that the companies, which were all involved in industrial control systems, were likely to have been targeted because of their access to Natanz.  The companies were identified as Foolad Technic Engineering Co., Behpajooh Co. Elec & Comp. Engineering, Neda Industrial Group, and Control Gostar Jahed.  In addition, Kala Electric, a producer of uranium enrichment centrifuges, was named Kaspersky Lab, a computer security firm, as a fifth company likely targeted by the computer worm.