Bacteriophage infecting bacterium, illustration. A bacteriophage, or phage, is a virus that infects bacteria. It attaches to the surface of a bacterium and injects its genetic material (yellow helix) 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. Phages are a possible therapy against multi-drug-resistant bacteria.