A virus that reproduces in bacterium is called a bacteriophage, or simply a phage. Bacteriophages are a type of virus that infects and replicates within bacteria. They are composed of a protein coat that encloses a DNA or RNA genome. Bacteriophages can be either virulent or temperate. Virulent phages lyse (break open) the host cell after replication, while temperate phages integrate their genome into the host cell's genome and become dormant.