Bacteriophages attacking bacteria, illustration. A bacteriophage, or phage, is a virus that infects bacteria. It consists of an icosahedral (20-sided) head, which contains the genetic material, a tail and tail fibres, which fix it to a specific receptor site on the bacterium. The tail injects the genetic material into the bacterium. These genetic instructions are used by the bacterium to build new bacteriophages. The new bacteriophages accumulate and burst out of the bacterium, killing it. Bacteriophage means 'bacterium eater' and they are now being used in phage therapy as an alternative to antibiotics to fight infection.