virus: (virology) ultramicroscopic infectious agent that replicates itself only within cells of living hosts; many are pathogenic; a piece of nucleic acid (DNA or RNA) wrapped in a thin coat of protein
microorganism: any organism of microscopic size
nuclear propulsion: the use of a nuclear reactor either to produce electricity to power an engine (as in a nuclear submarine) or to directly heat a propellant (as in nuclear rockets)