The Sobig C worm/virus is now being widely circulated via spam, but it's starting to look like a plot. "The Sobig.C variant is programmed to expire on June 8 and Sobig.C was released on the same day that its predecessor, Sobig.B, was programmed to stop spreading.The serial releases may be an effort by the Sobig author to fool antivirus software by subtly altering the makeup of the virus. Alternatively, the author could be releasing "proof of concept" viruses, testing the success of different viruses depending on when and how they are distributed," reports PCWorld.com.