For now, the key plans for nanotech are biological. Many scientists believe that it can and will protect against diseases by being assimilated into the bloodstreams as microscopic robots. These nanobots would search the body for tell-tale symptoms of terminal diseases, report findings to a computer and stop them before they manifest by reconstructing defective tissue, destroying irreparable tissue and this is done virtually automatically. Note that nanobot are robotics measured by nanometers (1mm= 1,000,000 nanometers), but most nanobots currently being studied are within the range of 0.1-10 micrometers, that is 0.0001-0.1mm.
Some scientists take it further by believing that it could prevent dying itself!