פירוש על נגעים 3:6

הקרחת והבדחת – further ahead in chapter 10 [Tractate Negaim, Mishnah 10], it (i.e., the Mishnah) explains what is leprous affection on the ack of the head and what is baldness on the forehead.

Scalp baldness or forehead baldness [negaim] become unclean for two weeks and by one of the following two signs: by quick flesh or by a spreading.
"By quick flesh," in the beginning, at the end of the first week, at the end of the second week, or after they have been pronounced clean.
"Or by a spreading," at the end of the first week, at the end of the second week, or after they have been pronounced clean.
They become unclean for two weeks which are only thirteen days.
Section one: Today's mishnah outlines the signs and length of impurity for negaim that appear on a bald head or a scalp (Leviticus 13:40).
The remainder of this mishnah has already been explained above in mishnayot 2-5.

במחיה ובפסיון (with quick/raw flesh and spreading) – but with white hair, they do not defile, because no hair grows in them.