Jesus said the yeast of the Pharisees is hypocrisy. This is worse than their being hypocrites. The warning is beware their stuff doesn't get inside you - in your head or in your heart. Once in it's like yeast (leaven) in bread. It permeates, infuses, and saturates. It's necessary to realize we are all being yeasted by someone or someones. I'm attempting to yeast you at this very moment. It's how we learn and change. All yeasting is not bad. Jesus' warning to all is Be on guard against the yeast of the Pharisees which is hypocrisy.