Picture of a phage virus Phage What is it Short for bacteriophage, phage is a virus that attacks a specific strain of bacteria – the ones we are concerned with attack the bacteria contained in your starter culture.  The way they work is to inject their DNA into a living cell which then copies itself and the cell  bursts releasing a number of phage, which go on to attack other cells.  Where the multiplication of the phage is greater than the (good) bacteria in your starter cul