"Your boasting about this is terrible. Don't you realize that this sin is like a little yeast that spreads through the whole batch of dough?" -1 Corinthians 5:6
Have you ever gotten involved in something only to realize a lot of it was hype? Marketing, promotion, and a large number of people believing in something, (even thought it may not be true), can make something ordinary seem remarkable, resulting in unrealistic expectations and false reality. It's the stuff celebrity obsession and cults are made of.
Jesus understood this leaven, and told people to watch out for it - especially when it came to the Pharisees. "Then Jesus said to them, 'Take heed and beware of the leaven of the Pharisees and the Sadducees.'" (Matthew 16:6 NKJV)
Jesus was always speaking in parables, and sometimes this got the disciples confused. They thought He was talking about bread! But then they finally understood He was talking about the "fluff" of the Pharisees.
If you think about leaven, it's purpose is to puff up bread, making it many times it's normal size. When Jesus was speaking of the Pharisees, he was speaking of the fact that they taught in such a way to make themselves look bigger, better, and more important than they really were. They highlighted their own importance, which disgusted Jesus, and made the people take their eyes off of God and onto them.
But Jesus was not into fluff. He was the real thing. The unleavened bread the Israelites ate during Passover was a prophetic sign of the coming of Jesus - the true unleavened bread.
Have you ever run into "fluff" in your life? Pray that God would help you to recognize it, and stay away from it. God desires we stay humble and close to the truth, which is found in Jesus.