Negaim, Chapter 8, Mishnah 2. This chapter, one of the more demanding in the tractate, revolves around prichah: the case in which the nega does not remain a spot but breaks out until it covers the entire body. A remarkable feature of this halachah is that the same total covering can cut in two opposite directions. When it comes upon a person who is already tamei, it releases him and he is tahor. Our mishnah teaches the reverse case.
The mishnah states: "Haporei'ach min hatahor, tamei." The subject here is a person who had already received a ruling of taharah from the kohen. Two paths could have brought him there: he may have been muchlat and then lost his simonei tumah, or he may have completed both periods of hesger without the nega expanding, and the kohen dismissed him as tahor. At that point the baheres erupts and covers him from head to foot. Such a prichah does not preserve the taharah he was holding; it overturns it and renders him tamei.
The mishnah continues: "Chazru bo roshei evarim, tamei." The roshei evarim, the twenty four extremities, are not susceptible to tumas nego'im, since their sloping surfaces cannot be seen by the kohen in one glance, and when a prichah covers the body they are absorbed into it. If they now come back into view with healthy flesh, michyah, upon them, the man is tamei. This is really no chiddush at all. If in the case where prichah brings taharah such a michyah is enough to render the person tamei, then certainly here, where the prichah has already left him tamei, the reappearance of the extremities is no mitigating factor whatsoever. He simply nishar betumaso, he remains exactly where he was.
What is his road back to taharah? The mishnah closes with "Ad shetachzor baharto lekemos shehaysah" - until the baheres shrinks back to the measurement it occupied beforehand. Observe how lenient the requirement is. There is no demand that the spot diminish below the size of a gris. All that is needed is that the pisyon vanish, with the baheres reduced once more to what it had been originally. With the spreading undone, the siman tumah generated by the prichah is undone along with it, and the man is tahor.
The lesson of the mishnah is that the very same phenomenon, a nega covering the whole body, is measured entirely by the status the person held when it arrived. Coming upon one who stood in taharah, it brings tumah, and only the retreat of the baheres to its original size can restore him.