Leviticus 13:20

And if, when the priest seeth it
And has thoroughly viewed it and considered it: behold, it [be] in sight lower than the skin;
having eaten into and taken root in the flesh under the skin: and the hair thereof be turned white;
which are the signs of leprosy before given, ( Leviticus 13:3 ) ; the priest shall pronounce him unclean;
not fit for company and conversation, but obliged to conform to the laws concerning leprosy: it is a plague of leprosy broken out of the boil;
which was there before: this is an emblem of apostates and apostasy, who having been seemingly healed and cleansed, return to their former course of life, and to all the impurity of it, like the dog to its vomit, and the swine to its wallowing in the mire, ( Proverbs 26:11 ) ( 2 Peter 2:22 ) ; and so their last state is worse than the first, ( Matthew 12:45 ) ( Luke 11:26 ) , as in this case; at first it was a boil, and then thought to be cured, and afterwards arises out of it a plague of leprosy.