Leviticus 13:19-20

19 and in the place where the boil was, a white swelling or reddish-white spot appears, he must present himself to the priest.

20 The priest is to examine it, and if it appears to be more than skin deep and the hair in it has turned white, the priest shall pronounce him unclean. It is an infectious skin disease that has broken out where the boil was.