1. Provided that technological research continues, nanotechnology will
eventually be developed.
2. An immune system wouldn't work unless it was global.
3. In the absence of a global immune system, if everybody could make
their own nanotech machines then all life on earth would soon become
extinct.
4. In the absence of ethical motives, the benefits would outweigh the
costs for a nanotech power that chose to eliminate the competition or
prevent it from arising, provided it had the ability to do so.
Nicholas Bostrom
http://www.hedweb.com/nickb