Midrash su Levitico 13:42

1) (Vayikra 13:9) ("A plague-spot of leprosy, if it be in a man, then he shall be brought to the Cohein. (Vayikra 13:10) And the Cohein shall see, and, behold, a white se'eth in the skin, and it has turned the hair white, and the healthiness (michyah) of living flesh within the se'eth"): "a plague-spot of leprosy": What is the intent of this? From "and, behold, a white se'eth" we learn that a se'eth becomes tamei with a michyah. Whence do we derive the same for the other appearances? (From "a plague-spot of leprosy.") — But does this not follow (even without the verse)? viz.: If we find all of the other appearances to be similar to se'eth vis-à-vis becoming tamei through white hair, let them be similar to se'eth vis-à-vis becoming tamei through michyah. And, furthermore, it follows a fortiori, viz.: If all of the appearances are similar to se'eth to become tamei through white hair, which does not confer tumah in karachath (back of the head) and gabachath (front of the head, viz. Vayikra 13:42-43), how much more so should they be similar to se'eth to become tamei through michyah, which does confer tumah in karachath and gabachath! — No, if all of the other appearances are similar to se'eth to become tamei through white hair, it is because white hair confers tumah in shechin (boils) and michvah (burns). Should they then be similar to se'eth to become tamei through michyah, which does not confer tumah in shechin and michvah! It must, therefore, be written "a plague-spot of leprosy" (to tell us that they do become tamei through michyah. And just as a se'eth is an um (a generator of leprosy), so is a bahereth an um. And whence do we derive the same for the other appearances? From the same a fortiori argument (as above) and from the same answer, viz. (It is derived) from "a plague-spot of leprosy."

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.

2) "blossoming": We are hereby taught that it confers tumah by spreading. "it": It does not confer tumah through white hair. For (had "it" not been written for this exclusion,) it would follow a fortiori (that it does confer tumah through white hair), viz.: If boil or burn, which do not confer tumah through michyah, do confer tumah through white hair, then karachath or gabachath, which do confer tumah through michyah, how much more so should they confer tumah through white hair! It is, therefore, written "it," to teach us that it does not confer tumah through white hair.