Midrash su Levitico 13:43

3) (Vayikra 13:43) "And the Cohein shall see him, and, behold, a rising of the plague-spot, reddish-white, (is in his karachath or in his gabachath"): We are hereby taught that a rising confers tumah intermixed. And whence (is the same) to be derived for the other appearances? From (Vayikra 13:42) "a reddish-white plague-spot in his karachath or in his gabachath … as in the appearance of the leprosy of the skin of the flesh." Just as this [karachath] confers tumah intermixed, so the leprosy of the skin of the flesh confers tumah intermixed, and just as the skin of the flesh confers tumah plain, non-intermixed, this, too, confers tumah plain, non-intermixed. "the skin of the flesh": (which requires a quarantine of) two weeks.

4) (Why is this Scriptural analogy [hekesh] needed?) Does it not follow by induction (mah matzinu)?, viz.: Tumah is conferred here (in the instance of karachath), and tumah is conferred in (the instance of) the skin of the flesh. Just as the skin of the flesh, two weeks, here, too, two weeks.

5) Or, perhaps go in this direction: Tumah is conferred here, and tumah is conferred in (the instance of) boil or burn. Just as boil or burn, one week, here, too, one week.