Computer Immune Systems

LL writes "We might soon be seeing commercial delivery of autoimmune security systems. Rather than the surface bit pattern detections of antivirus checkers, these system attempt to provoke virii in a secure area (IBM) or match network packets against signature tags (Forrest). The interesting plug is that the author suggests that large programs such as operating systems should be made in such a way that no two copies are exactly alike. Now guess what favourite beast has this trait?"