Midrash su Levitico 13:11

1) (Vayikra 13:42) ("And if there be on the karachath or on the gabachath a reddish-white plague-spot, it is blossoming leprosy on his karachath or on his gabachath.") "a reddish-white plague-spot": We are hereby taught that it confers tumah intermixed. "leprosy": We are hereby taught that it confers tumah through (the eruption of) a michyah (viz. Section 4, Chapter 7:9), (michyah also being called "leprosy" [viz. Vayikra 13:11]). For (had "leprosy" not been written in this connection,) it would follow a fortiori (that it does not confer tumah through a michyah), viz.: If boil or burn, which do confer tumah through white hair, do not confer tumah through michyah, then karachath or gabachath, which do not confer tumah through white hair, how much more so should they not confer tumah through michyah! It is, therefore, written "leprosy," teaching us that it does confer tumah through michyah.

1) (Vayikra 13:11) ("It is old leprosy in the skin of his flesh. And the Cohein shall declare him tamei; he shall not quarantine him, for he is tamei.") "leprosy": This is a prototype (binyan av) for all leprosy as requiring a garis (the size of a Cilician bean, for tumah). "old": We are hereby taught that the michyah confers tumah if it preceded ([hence "old"] the se'eth). For (without the verse) would it not follow (that it should not do so, viz.): White hair is a sign of tumah, and a michyah is a sign of tumah. Just as white hair confers tumah only if it did not precede the se'eth, so michyah should confer tumah only if it did not precede the se'eth. It is, therefore, written "old," to teach us that it confers tumah if it preceded the se'eth.

2) "It (is leprosy"): This teaches us that it (also) confers tumah if it did not precede the se'eth. Now (even without the verse) would it not follow a fortiori that it does so, viz.): If white hair, which does not confer tumah if it precedes the se'eth (does confer tumah if it follows the se'eth), then michyah, which does confer tumah if it precedes the se'eth, how much more so should it confer tumah if it follows the se'eth! … It must, therefore, be written "It," to teach us that it confers tumah if it follows the se'eth.