T4 bacteriophage. 3D computer illustration of a T4 bacteriophage virus before in the attachment position (left) and one in the injecting position (right). A bacteriophage, or phage, is a virus that infects bacteria. Enterobacteria T4 infects E. coli bacteria. It consists of an icosahedral (20-sided) head, which contains the genetic material, a tail (cylinder) and tail fibres (leg-like). The tail fibres attach to the surface of the bacterium and the tail injects the genetic material into the cell. The viral genetic material then 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 burst out of the cell, killing it in the process.