Rashi on Leviticus 13:11/רש"י על ויקרא י״ג:י״א

1צרעת נשנת הוא IT IS AN OLD LEPROSY — i. e. it is an old leprous malady beneath the sound flesh, and this wound appears healthy above, but underneath it is full of moisture (pus), therefore the priest shall pronounce it unclean. Scripture states this in order that you should not say: since healthy flesh has come over it I shall pronounce it clean.