Bacteriophages 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 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.