Rashi on Leviticus 13:40/רש"י על ויקרא י״ג:מ׳

1קרח הוא טהור הוא HE IS BALD, HE IS CLEAN — clean from the uncleanness of נתק (a scall in a hairy spot) (Sifra, Braita d'Rabbi Yishmael 15), for he is not subject to the law about the symptoms of the head or the beard, which are places where hair grows, but to that about the symptoms of a plague on the skin of the flesh — [white hair], healthy flesh or the spreading of the plague.