Negaim, Chapter 3, Mishnah 5. This perek sets out, category by category, how long each type of nega is examined and which signs make it tamei. Our Mishnah deals with negaim that arise in a place where the skin was lost through a wound, and it turns out to be the exception to the general pattern: a single week of confinement rather than two.
The six categories of negaim
Rambam, in the comment that closes this chapter, lays out the full map for us. There are six kinds of negaim altogether. Four of them appear on a human being: nigei or basar, on the plain skin of the flesh; nigei shechin u'michvah, where the skin has been stripped away; nigei rosh v'zakan, in the areas that grow hair on the head and in the beard, which are given the name nesakim; and nigei karachas v'gabachas, on a bald head, gabachas being the bald patch toward the face and karachas the one toward the nape. A fifth kind is nigei begadim, the negaim of clothing, and all five of these are recorded in Parshas Tazria. The sixth is nigei batim, the negaim of houses, taught in Parshas Metzora, right after the section describing how a human metzora is purified and which korbanos he brings. Each of the six has simanim of its own, and there is one siman shared by every one of them: hapisyon, spreading. Pisyon never operates at the opening inspection; it begins to be metamei only once a first week of hesger has run its course, and from that point onward.
Shechin and michvah
"Hashechin v'hamichvah": we are speaking of a spot of exposed raw flesh, where the covering of skin has been destroyed. What decides which of the two names applies is nothing but the cause of the injury. Damage produced by heat, whether an open flame, a glowing coal, a smoldering ember or a heated piece of metal, is called a michvah. Damage from any other source, a blow that tore the skin off or an illness that ate it away and left the flesh bare, is called a shechin. To the eye the two wounds may be indistinguishable, yet halachah keeps them apart: a shechin and a michvah are not mitztaref with one another to make up the shiur of a gris.
One week, two signs
"Mitamin b'shavua echad u'vishnei simanim." Here is the exception in our perek. Every other category of nega on a person runs for two weeks; nigei batim, the negaim of houses in the closing Mishnah of the perek, run for three. Shechin and michvah are confined for one week only, and only two signs can render them tamei: seir lavan, white hair, and pisyon, spreading. There is no michyah in this category.
Seir lavan means that hairs which had been black in that patch have turned white. "B'seir lavan, bitchilah u'v'sof shavua u'le'achar hapetur." White hair is effective at three points: bitchilah, at the very first examination, so that if on the very day the man comes before the kohen the kohen sees that the black hairs there have turned white, he is at once a metzora muchlat; at the end of the single week of confinement; and le'achar hapetur, after the kohen has already pronounced him tahor and sent him away, if the white hairs then suddenly appear.
Pisyon means that the nega has spread beyond its original borders. "U'v'pisyon, b'sof shavua u'le'achar hapetur." Spreading is metamei at only two points: at the end of the week of confinement, and after the petur, if he had been declared clean and the nega then spreads. Pisyon cannot serve at the first examination, since at that moment there is nothing yet from which the nega could have spread.
Seven days
"U'mitamin b'shavua echad, shehu shivas yamim." The Mishnah concludes by telling us that the hesger for shechin and michvah lasts one week, seven days. If the signs of tumah do not present themselves, whether the white hair at the outset or at the close of the week, or the spreading at the close of the week, the man is dismissed as tahor. This is the one category in which the entire examination is compressed into a single week.