Scientists have developed what they say could become the world's smallest medical kit: a computer, made of DNA, that can diagnose disease and automatically dispense medicine to treat it. The computer, so small that one trillion would fit into a drop of water, now works only in a test tube, and it could be decades before something like it is ready for practical use. But it offers an intriguing glimpse of a future in which molecular machines operate inside people, spotting diseases and treating them before noticeable symptoms even appear.