Computer viruses are sneaky, omnipresent and out of control -- but are they alive? The British physics giant Stephen Hawking thinks so. In a speech given at Apple Computer's Macworld Expo in Boston, the wheelchair-bound author of "A Brief History of Time" mused that computer viruses meet all the definitions of life, except for the fact that they have no metabolism. Instead, Hawking said, they're parasites that feed off the metabolisms of the host computers they destroy: "I think it says something about human nature that the only form of life we have created so far is purely destructive. We've created life in our own image."