A bacteriophage, also known informally as a phage , is a virus that infects and replicates within bacteria and archaea. The name was derived from "bacteria" and the Greek φαγεῖv, meaning "to devour". Bacteriophages are made up of proteins that encase a DNA or RNA genome and can have simple or complex structures. Their genomes might contain as little as four genes or as many as hundreds. Following the injection of their DNA into the cytoplasm of the bacterium, phages proliferate within it.