Jesus warns us to be on guard not only against Satan, temptations, enemies, and persecutors, but also against yeast (of all things). When Jesus initially gave this warning, His disciples were confused. They thought He was literally speaking of yeast used to make bread (Mk 8:16). However, Jesus was speaking of "the yeast of the Pharisees and the yeast of Herod" (Mk 8:15). He was speaking of the yeast of hostility against Him, hypocrisy (Lk 12:1), and false teaching (Mt 16:12). Jesus is warning us that warped and lukewarm Christianity can be contagious.
We should not respond to His warning by going in search of the perfect church. Rather, we are to respond by overcoming evil with good (Rm 12:21), fighting tepidity with fire (see Rv 3:16), and unmasking false teaching by speaking the truth in love (Eph 4:15).
There is not only a corrupting yeast but also a renewing one. Not only is the kingdom of Satan like yeast, but also "the kingdom of God is like yeast which a woman took and kneaded into three measures of flour. Eventually the whole mass of dough began to rise" (Mt 13:33, our transl). Not only can evil be contagious, but so can faith. Faith is not only taught but caught. If you guard against the dangers of the yeast of corruption and wickedness (1 Cor 5:8), Satan himself will be unable to stop the rising power of your yeast of renewal and holiness. The kingdom of God is like the yeast of God's kingdom.