False-colour transmission electron micrograph of a T4 bacteriophage (red), a virus that infects only bacteria; T4 is a specific parasite of Escherichia coli. Phages are found in sewage, polluted water and faeces. T4 attaches itself to the cell wall of E.coli with its spidery tail fibres, visible here. The ribbed elongated tail is a contractile sheath, which contracts to squirt the contents of the head, the genetic material, into the host cell. T4 DNA commandeers the genetic machinery of the host, forcing it to code for the reproduction of T4 progeny. Magnification: x94,500 at 6x4.5cm size.