Tractate Negaim - Chapter 8 - Mishnah 5 נגעים פרק ח׳ משנה ה׳

Any part [of the body] that can be subject to the uncleanness of a bright spot nega can prevent the effectiveness of the breaking out [over the entire body], and any part that cannot be subject to the uncleanness of a bright spot nega does not prevent the effectiveness of the breaking out abroad. How so? If it broke out covering all of his skin, but not on the head or the beard, or on a festering boil, burn or blister, and then the head or the beard became bald, or the boil, burn or blister turned into a scar, he is clean. If it broke out covering all of his skin, except a spot of the size of half a lentil near the head or beard, or near a boil, burn or blister, and then the head or the beard became bald, or the boil, burn or blister turned into a scar, even though the place of the quick flesh became a bright spot, he is unclean unless it breaks out covering all his body.