Color enhanced transmission electron micrograph (TEM) of DNA from a bacteriophage. A bacteriophage is a virus that invades bacterial cells. The phage attaches to a bacterium's cell wall and then punches a hole in the cell wall and injects its DNA into the bacterium. The viral genetic material commandeers the bacterium's cellular machinery and instructs it to begin making new virus parts. These parts assemble to form new viruses, which eventually burst the bacterium and invade other cells.