Galatians 5:9

Everyone knew how people made bread. A baker has a big bowl of flour. He adds a very small piece of yeast. He mixes in a lot of water. Then he leaves the mixture in a warm place before he bakes it. Yeast is a substance that people use in bread. It produces a lot of air when it is warm and wet. Each loaf only has a small piece of yeast in it. But the whole loaf grows to about three times its original size.

This is what Paul meant. The false teachers may not seem very bad. But what they taught would spread. And it would affect the whole *church. And it would cause great damage.