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.