Midrash su Levitico 13:39

1) (Vayikra 13:39) ("Then the Cohein shall see, and, behold, if in the skin of their flesh there are dim white bright spots, it is bohak (white scurf) that has blossomed in the skin; it is clean.") "it is bohak … it is clean": This teaches us that bohak is clean (and does not require quarantine). I might think that it does not confer tumah through its source, but it does confer tumah by spreading; it is, therefore, written "that has blossomed (i.e., spread) in the skin; it is clean." I might think that it renders clean a bahereth that issued from it; it is, therefore, written "it" (bohak, is clean, but not a bahereth that issues from it). I might think that it does not render clean a bahereth that issues from it, but it does render clean a bahereth that is adjacent to it; it is, therefore, written "it is clean" but a bahereth or a spreading that either issues from it or is adjacent to it is not clean.

2) Whence is it derived that if he had a nethek the size of a garis and it came to cover his entire head, he is clean? From (Vayikra 13:40) "And a man, if the hair of his head falls out … he is clean." I might think that the beard (if it does not bald) prevents (the head from being tahor if it balds) and that this follows a fortiori, viz.: If the skin of the face and the skin of the (rest of) the flesh, which are separated by something else (the beard) prevent each other (from becoming tahor [if one of them does not turn white, so that it can be considered "all white"]), then the (skin of the head and the beard, which are not separated by anything — how much more so should they prevent each other (from becoming tahor if one of them balds but not the other)! It is, therefore, written "his head" — (Even if) his head (alone balds), and not his beard, "he is clean."

3) I might think that every appearance of snow is tamei and that all other appearances are tahor. It is, therefore, written (Vayikra 13:39) "It is a bohak (brightness) … it is clean": It is clean, but everything above it (in brightness) is tamei.