T bacteriophage Coloured transmission electron micrograph TEM of a T bacteriophage virus The swollen structure at top is the head which contains DNA inside a protein coat Attached to this is the tail consisting of a tubelike sheath and tail fibres at bottom T bacteriophages are parasites of Escherichia coli a bacteria common in the human gut The virus attaches itself to the host bacteria cell wall by its tail fibres the sheath then contracts injecting the contents of the head DNA into the host The viral DNA makes the bacteria manufacture more copies of the virus Magnification x at xcm size