T4 bacteriophage. computer artwork of a T4 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 tube-like sheath and tail fibres (at bottom). T4 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.