Negaim, Chapter 8, Mishnah 1. We are opening a chapter that is among the harder ones in the tractate. Its subject, almost from beginning to end, is prichah: the striking situation in which the nega does not stay in one place but breaks out and takes over the person's whole body. The surprising rule of the Torah is that such a total covering can work in two opposite directions, and this opening mishnah teaches the first of them.
The mishnah begins: "Haporei'ach min hatamei, tahor" - one in whom the nega breaks out over his entire body while he is in a state of tumah becomes tahor. The Rav describes the case as a tamei shenechlat, someone whom the kohen had already declared muchlat on the basis of one of the signs of nego'im. If the baheres then spread until it covered him completely, that very spreading releases him from his tumah.
The Acharonim, among them the Tosfos Yom Tov and the Mishnah Acharonah, point out that the Rav's wording should not be taken as a limitation. It is lav davka a muchlat. Even a musgar, someone whom the kohen shut up for observation, is included. Hesger is itself a status of tumah, less severe than hechlet but tumah nonetheless. So whether the prichah came upon a muchlat, or came upon a musgar at the close of his first week or his second week, the ruling is the same: haporei'ach min hatamei tahor.
The mishnah then removes one case from this leniency: "Chazru bo roshei evarim, tamei." Here we need a point taught earlier in the tractate. The roshei evarim, the twenty four extremities of the body, are not susceptible to tumas nego'im, because their sloping surfaces cannot be taken in by the kohen's eye in a single view. When a prichah covers the entire body, these extremities are swallowed up along with everything else. Now the mishnah speaks of a case where those extremities came back into view, and healthy flesh, michyah, is seen on them. That michyah is a full siman tumah, and the man is tamei. In other words, prichah is powerful, but it is no answer to tumas michyah.
What, then, remains for him? The mishnah answers: "Ad shetimaet habaheres mikagris" - until the baheres becomes smaller than a gris. Since the michyah sits within the baheres, the way back to taharah is for that baheres to shrink below the minimum measure, so that it is no longer a baheres capable of conveying tumah at all.
So this mishnah gives us both sides of the coin in a single line. A total outbreak upon one who is already tamei is his release, and yet the reappearance of the roshei evarim with living flesh upon them pulls him straight back into tumah, from which only the shrinking of the baheres itself can free him.