Term: bacteriophage: a virus that is parasitic (reproduces itself) in bacteria
Broader: 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 | Unrelated: doctor: children take the roles of physician or patient or nurse and pretend they are at the physician's office