Bacteriophage on bacterium. Illustration of a bacteriophage injecting its genetic material (yellow and red strand) into a bacterium. A bacteriophage, or phage, is a virus that infects bacteria. It consists of a head (top) containing the genetic material, a tail (central section) and tail fibres, which fix it to a specific receptor site. The tail injects its genetic material into the bacterium through the cell membrane, and this hijacks the bacterium's own cellular machinery, forcing it to produce more copies of the bacteriophage. When a sufficient number have been produced, the phages exit the cell by lysis, killing it in the process.