A bacteriophage (informally, phage) is a virus that infects and replicates within bacteria. The term is derived from ‘bacteria‘ and the Greek ?????? phagein “to devour”. Bacteriophage are composed of proteins that encapsulate a DNA or RNA genome, and may have relatively simple or elaborate structures. Their genomes may encode as few as four genes, and as many as hundreds of genes. Phage replicate within bacteria following the injection of their genome into the cytoplasm.