Midrash su Levitico 13:21

1) (Vayikra 13:19) ("and there be in the place of the boil a white se'eth or a reddish-white bahereth, then it shall be shown to the Cohein. (Vayikra 13:20) And the Cohein shall see, and, behold, its appearance is lower than the skin, and its hair has turned white, then the Cohein shall declare him unclean. It is a plague-spot of leprosy; it has blossomed in the boil. (Vayikra 13:21) And if the Cohein see it, and, behold, there is no white hair in it, and it is not lower than the skin, and it is dim, then the Cohein shall quarantine him for seven days.") "and there be in the place of the boil a white se'eth": The boil must precede the se'eth, and not the se'eth, the boil. R. Eliezer b. Yaakov says: It was called "its place" (that of the bahereth) before it (the boil) arrived there.

4) (Vayikra 13:20) "its appearance is lower": This tells me (that it is tamei) only if its appearance is lower. Whence do I derive for inclusion (an appearance that is) level (with the skin) or higher? From (Vayikra 13:21) "and it is not lower than the skin," (which indicates that "lower than the skin" is not a categorical requirement for tumah). "it has blossomed in the boil": but not in the skin of the flesh (outside of the boil). I might think that (to become tamei) it does not spread in the skin of the flesh, but it does spread in the skin of the burn (near the boil); it is, therefore, written (Vayikra 13:23) "And if in its place the bahereth stands — (if) it has not spread." In its place (in the boil) it spreads, (to become tamei); it does not spread in the skin of the flesh or in the skin of the burn.

5) (Vayikra 13:21) "And if the Cohein see it" — all in one sighting. "and, behold, there is no white hair in it" — not in it (the boil), but not in a strand projecting from it (i.e., if there were white hair in the strand, it is tahor). I might think (that the strand was not considered part of the boil) even if there were in it the breadth of two hairs; it is, therefore, written ([Vayikra 13:26] in respect to a burn) "and, behold, there is not in the bahereth white hair" (connoting that any significant projection [i.e., the breadth of two hairs] is considered part of the bahereth).