What does the ‘risk of phage’ mean?

Fast acid cheeses, such as cheddar and related types require the starter bacteria to grow quickly to produce acid. If bacteriophage (known as phage, a virus which attacks bacteria) is present, then the rate of acidification can be severely reduced, producing poor quality cheeses. Home cheese makers, making a cheese weekly, for example, will not be at risk. Factories making several vats a day will be at risk. Starter rotation is a mechanism for reducing the risk.