Midrasz do Kapłańska 13:3

1) (Vayikra 13:3) ("And the Cohein shall see the plague-spot in the skin of the flesh. If hair in the plague-spot has turned white, and the appearance of the plague-spot is deeper than the skin of his flesh, then it is a plague-spot of leprosy; and the Cohein shall see it and he shall declare him unclean.") "and the Cohein shall see the plague-spot": His eyes should be intently upon it when he inspects it. "in the skin of the flesh": intermediate (i.e., even if he is an Ethiopian, he is seen as "intermediate" [viz. Chapter 1:5]). "in the skin of the flesh": He should inspect all the flesh with it as one (to see if it has spread). R. Yossi b. R. Yehudah says: The nega must be outside the skin of the flesh, right next to it, so that it lends itself to spreading. For if it were right next to the head, the beard, or a "rebellious" boil or burn or inflammation (viz. Chapter 1:1), it would not be tamei.

2) "If hair in the plague-spot": at least two hairs. "in the plague-spot": to include (as tamei) a hair rooted in the nega and lying outside of it; to exclude a hair rooted outside the nega and lying in it. "has turned white": and not (if it was white) before (the appearance of the plague-spot). From here they ruled: If the bahereth preceded the white hair, he is tamei; if the white hair preceded the bahereth, he is tahor; if there is a doubt (as to which preceded which), he is tamei. R. Yehoshua says: It is "dim" (and he is to be quarantined).

3) "If the hair in the plague-spot has turned white": From here they ruled: (If there were) two hairs, their roots ("in the plague-spot") black and their tops white, he is tahor; their roots white and their tops black, he is tamei. How large must the white be (to render him tamei)? R. Meir says: Any size. The sages say: The standard size (i.e., large enough for the scissors to cut). R. Meir says: Men should not think that they are judged by the whole hair, but if the root of the hair ("in the plague-spot") is white, he is tamei; and if it is not white, he is tahor.