Midrash su Levitico 13:30

1) (Vayikra 13:29) ("And a man or a woman — if there be in it a plague-spot, in the head or in the beard (Vayikra 13:30) then the Cohein shall see the plague-spot; and if its appearance is deeper than the skin and there is in it thin yellow hair, then the Cohein shall declare him tamei. It is a nethek (a scall); it is leprosy of the head or the beard.") "And a man": to include a nethek within a nethek (as being tamei). "a man or a woman": This tells me only of a (distinct) man or woman. Whence do I derive for inclusion a tumtum (one whose sex is in doubt) or a hermaphrodite? From "or." "if there be in it a plague-spot in the head or in the beard": This teaches us that the head and the beard do not combine with each other (vis-à-vis the minimum size for tumah).

2) I might think that though they do not combine with each other, they can spread from one to the other (vis-à-vis "spreading" uncleanliness); it is, therefore, written (Vayikra 13:30) "leprosy of the head or the beard" — Just as they do not combine with each other, so they do not spread from one to the other.