The original magic bullet was an idea expressed by Paul Ehrlich, of a drug targeted precisely on Syphilis. Since then it has been imitated greatly, and applied in many circumstances. It is a useful concept. Modern understanding of physics and immunology has given us the capacity to make a variety of them. A command of nanotechnology in the sense proposed by Drexler would give us nearly unlimited magic bulletting capabilities.
[Foresight Institute http://www.foresight.org/]