Coloured scanning electron micrograph (SEM) of bacteriophages (yellow) attacking a Pseudomonas aeruginosa bacterium (orange). A bacteriophage, or phage, is a virus that infects bacteria. The genetic material is contained within the head (round) of the phage. The phage attaches to the surface of a bacterium and injects its 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. Magnification: x14,825 when printed at 10 centimetres tall.