Viruses are beautiful, simple, elegant, and deadly machinery. They are not alive for the same reason that a computer virus is not alive. It does not deviate from it's programming unless mutation arises. It's sole purpose is to propagate. It does not react to any stimuli. I poke you with a hot iron, you'll jump back, I attack a virus, it'll just die. The key is knowing how to attack it in the right fashion, and leaving no survivors, to prevent mutation.