Negaim, Chapter 3, Mishnah 4. Until now our masechta has given us general introductions, and the mishnayos have referred to the different categories of negaim only by allusion, through the various klalim and rules they have laid down. Here the Mishnah begins to spell out, category by category, how long each kind of nega is examined and which signs render it tamei.
The six categories of negaim
The Rambam, in his closing remarks to this perek, lays out the entire map for us. Negaim come in six varieties. Four of them can strike a person's body: nigei or basar, which arise on the plain skin of the flesh; nigei shechin u'michvah, which arise in a spot where skin has already been destroyed by injury (michvah refers to skin lost through heat, whether a flame, heated metal, a coal or an ember, while shechin covers skin lost by any other means, a bruise or an illness that leaves the flesh raw); nigei rosh v'zakan, which come up in the hair-bearing regions, that is, the scalp and the area of the beard, referred to in the later mishnayos as nesakim; and nigei karachas v'gabachas, the negaim of a bald patch, karachas being the rear of the scalp and gabachas the portion toward the front. A fifth variety the Rambam lists is nigei begadim, negaim that appear on clothing. All five of these are taught in Parshas Tazria. The sixth, nigei batim, negaim that afflict houses, is written in Parshas Metzora, where it comes after the Torah's account of how a human metzora is purified and which korbanos he brings.
Every one of these varieties, the Rambam goes on, carries its own set of simanim, and once such a siman becomes visible, tumah takes hold, whether on the man or on the object. There is, however, one siman shared by all six without exception: hapisyon, the spread of the nega. And pisyon comes with a restriction attached. It never counts at the opening inspection; it operates only from the conclusion of the initial week of hesger and onward. Our Mishnah itself will make this plain.
The Mishnah
"Or habasar mitamei bishnei shavuos u'vishlosha simanim": a nega on the ordinary skin of the flesh is examined over a span of two weeks, and it can become tamei through three different signs. The three are seir lavan, michyah and pisyon. Each of them will be treated at length in the prakim that follow; here the Mishnah is giving us the outline.
Seir lavan means that hairs which had been black within the white patch have turned white. Michyah is a patch of live, healthy flesh inside the nega: as we have learned, it must be a full square area, not scattered specks, and it must sit in the middle of the original discoloration, whether that discoloration is white or a white-and-reddish mixture. Pisyon means that the nega has spread beyond its original borders.
When each sign is metamei
The Mishnah continues: "b'seir lavan u'v'michyah", by white hair and by michyah. These two are effective at four separate junctures. The first is "bitchilah", right at the start: should the kohen, when the man first presents himself, find white hairs or a michyah, he pronounces him a metzora muchlat then and there, and the man departs to dwell outside the camp. The second is "u'v'sof shavua rishon", when the opening week of hesger runs out. The third is "u'v'sof shavua sheini", once the following week has closed. The fourth is "u'le'achar hapetur", after both weeks have passed and the kohen has already found grounds to release him as tahor: if a day later, or two days later, or even considerably later, white hairs sprout or a michyah surfaces for the first time, he is tamei.
With pisyon the Mishnah lists only three junctures: "u'v'pisyon", spreading is metamei "b'sof shavua rishon", when the opening week of hesger concludes, "u'v'sof shavua sheini", when the following week concludes, and "u'le'achar hapetur", any time after his release as tahor. What spreading cannot do is render him tamei at the very first inspection, since at that moment there is no earlier boundary that the nega could have grown past. The upshot is this: seir lavan and michyah take effect from the very outset, whereas pisyon enters the picture only once that initial week has run its course.
Thirteen days
"U'mitamei bishnei shavuos, shehein shelosha asar yom." The Mishnah signs off by informing us that this pair of weeks adds up to thirteen days rather than fourteen. The reason is a rule we have already encountered: day seven counts twice, closing out the initial stretch of hesger while simultaneously opening the stretch that follows. Two weeks of confinement for or habasar therefore total thirteen days, and it is across that span, together with the first inspection itself and the period following a ruling of taharah, that the three simanim of nigei or basar do their work.