Mark 8:15 "And He was giving orders to them, saying, "Watch out! Beware of the leaven of the Pharisees and the leaven of Herod.""
The leaven of the Pharisees I think we know about: the sense that our works or our position make us holier that other people. The effect isn't limited to Christians or Jews, I can easily name atheists who are holier-than-thou--for their definition of holy. It is an easy sense to acquire--we have to try to be holy, and be alert about problems, and you notice pretty quickly that some people aren't.
I looked around for what people though the leaven of Herod might be.
Mark 6:20 "Herod was afraid of John, knowing that he was a righteous and holy man, and he kept him safe. And when he heard him, he was very perplexed; but he used to enjoy listening to him."
Luke 23:8 "Now Herod was very glad when he saw Jesus; for he had wanted to see Him for a long time, because he had been hearing about Him and was hoping to see some sign performed by Him." (Jesus refused to say a word to him.)
Ezekiel 33:32 "Behold, you are to them like a sensual song by one who has a beautiful voice and plays well on an instrument; for they hear your words but they do not practice them."
Jesus referred to Herod as a fox, and I gather that the fox was not a symbol of cunning in that place and era. One thinks of Laodacea.
The descriptions aren't very detailed, but I get the impression of a man who liked to hear the word of God. Maybe even loved it, but didn't love it enough to do anything about it.
So, maybe He meant the leaven of connoisseur-ship?