Negaim, Chapter 3, Mishnah 6. Our chapter sets out the six types of nega'im: four kinds that come upon a person, one that appears in clothing, and one that appears in a house. After treating the nega'im of plain flesh and then those that arise upon a boil or a burn (shechin and michvah), the Mishnah moves on to the nega'im found in those parts of the body that grow hair.
In his introduction to these mishnayos the Rambam labels this group nig'ei harosh v'hazakan, the nega'im of the head and of the beard. Scripture, and the Mishnah after it, use a different term: hanesakim. Hence the Rav's opening clarification, that a nega appearing on the scalp or within the beard goes by the name nesek.
Two weeks and two signs
The Mishnah teaches: "Hanesakim mitam'in bishnei shavu'os uvishnei simanim: b'sei'ar zahov dak uv'fisayon." Nesakim bring tumah upon a person across a stretch of two weeks, and by way of two signs: a fine golden hair, and spread.
Two weeks is the ordinary framework for the nega'im that come upon a person. Only one case departs from it, the nega that develops upon a boil or a burn, shechin and michvah, whose hesger is limited to a lone week. Nesakim belong to the usual scheme of two weeks.
What a nesek looks like
The character of this nega deserves a moment's attention. A nesek is a change of color in a hair-covered region, and it is not restricted to the eight appearances taught in the first chapter, namely the four grades of white running from snow-bright down to the palest membrane-like tint, along with their reddish blends. In hairy areas a discoloration of other colors can qualify as a nesek as well, although here too the eight mar'os are what one usually encounters. Tumah in a nesek is not determined by the shade at all, but by the pair of simanim the Mishnah names: a hair within the patch that was black and has turned a fine golden color, and the enlargement of the patch itself.
When each siman takes effect
The Mishnah goes on: "B'sei'ar zahov dak bit'chilah, uv'sof shavua rishon, uv'sof shavua sheni, l'achar hapetur." There are four moments at which the fine golden hair conveys tumah. Bit'chilah refers to the very first inspection: the Rav writes that should a golden hair already be present in the discolored area when the person first comes before the kohen, the kohen pronounces him a metzora muchlat immediately. Failing that, the hair can emerge when the first seven days of hesger are up, or when the second seven days are up. And even after the kohen has declared him tahor, should a fine golden hair make its first appearance later on, l'achar hapetur, he becomes a metzora muchlat.
"Uv'fisayon b'sof shavua rishon, b'sof shavua sheni, l'achar hapetur." Spread, by definition, cannot serve as a siman at the very first inspection, because there is no earlier measurement against which to judge it. It brings tumah only when the first stretch of seven days concludes, when the second stretch concludes, or once the kohen has already dismissed him as tahor.
Thirteen days
The Mishnah concludes: "U'mitam'in bishnei shavu'os shehein shloshah asar yom." The entire process of hesger, which finishes either in a petur or in the man being declared muchlat, spans two weeks, and those two weeks come to thirteen days, not fourteen. The reason is that day seven plays two roles at once: it is the day of the kohen's second inspection, and it simultaneously counts as day one of the following week. Seven days plus six more yields thirteen.