bacteriophage, also known informally as a phage, is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" meaning "to devour". Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have structures that are either simple or elaborate. Their genomes may encode as few as four genes (e.g. MS2) and as many as hundreds of genes. Phages replicate within the bacterium following the injection of their genome.