Luke 13:20,210  Again he said, "To what shall I compare the Kingdom of God?  It is like yeast, which a woman took and hid in three measures of flour, until it was all leavened."  (web)

"Yeast" or "Leaven" almost always has a negative connotation in the Bible. Jesus said, "Be on your guard against the yeast of the Pharisees, which is hypocrisy." Luke 12:1. And in another place he equates the leaven with the false teachings of the Pharisees. "B'e on your guard against the yeast of the Pharisees and Sadducees.'  Then they understood that he was not telling them to guard against the yeast used in bread, but against the teaching of the Pharisees and Sadducees." Mt 16:11,12

And Paul writes:"Your boasting is not good. Don't you know that a little yeast works through the whole batch of dough? Get rid of the old yeast that you may be a new batch without yeast-- as you really are. For Christ, our Passover lamb, has been sacrificed. Therefore let us keep the Festival, not with the old yeast, the yeast of malice and wickedness, but with bread without yeast, the bread of sincerity and truth." 1Cor 5:6-8

And he speaks similarly in the book of Galatians warning of the doctrine of the cult of the circumcision saying, "A little yeast works through the whole batch of dough." Gal 5:9

So from these passages we can clearly see the apostle's understanding of Jesus' parable. There are those who think and even preach that Jesus was using a positive connotation. They think that Jesus was saying that He is the yeast that makes the dough rise. But they are mistaken. The woman represents the devil.

False teachings and false brethren (Gal 2:4) make for a bloated church full of hot air - a church with a yeast infection.