Midrash su Levitico 13:19

1) (Vayikra 13:19) ("and there be in the place of the boil a white se'eth or a reddish-white bahereth, then it shall be shown to the Cohein. (Vayikra 13:20) And the Cohein shall see, and, behold, its appearance is lower than the skin, and its hair has turned white, then the Cohein shall declare him unclean. It is a plague-spot of leprosy; it has blossomed in the boil. (Vayikra 13:21) And if the Cohein see it, and, behold, there is no white hair in it, and it is not lower than the skin, and it is dim, then the Cohein shall quarantine him for seven days.") "and there be in the place of the boil a white se'eth": The boil must precede the se'eth, and not the se'eth, the boil. R. Eliezer b. Yaakov says: It was called "its place" (that of the bahereth) before it (the boil) arrived there.

2) How (is the verse to be understood)? There was (a se'eth) in the skin of the flesh before the arrival of the boil (in that place). R. Eliezer b. Yaakov ruled it (a se'eth that preceded the boil) tamei, and the sages ruled it tahor.

3) "a white se'eth": We are hereby taught that it confers tumah plain (i.e., without intermixture of red). "a reddish-white bahereth": We are hereby taught that it confers tumah intermixed. I might think that the se'eth confers tumah only plain (white) and the bahereth confers tumah only intermixed. Whence do I derive that what applies to se'eth (plain) applies to bahereth, and what applies to bahereth (intermixed) applies to se'eth? From (Vayikra 13:20) It is a plague-spot of leprosy" (connoting interchangeability).