Jamieson, Fausset and Brown on Galatians 5:9

9. A little leaven--the false teaching of the Judaizers. A small portion of legalism, if it be mixed with the Gospel, corrupts its purity. To add legal ordinances and works in the least degree to justification by faith, is to undermine "the whole." So "leaven" is used of false doctrine (Mt 16:12: compare Mt 13:33). In 1Co 5:6 it means the corrupting influence of one bad person; so Bengel understands it here to refer to the person (Ga 5:7, 8, 10) who misled them. Ec 9:18, "One sinner destroyeth much good" (1Co 15:33). I prefer to refer it to false doctrine, answering to "persuasion" (Ga 5:8).