Nazir, Chapter 7, Mishnah 2. This mishnah addresses the question of which types of tumah obligate a nazir to undergo the process of shaving his head and bringing offerings for his tumah. Not every form of tumah brings this about: first, it must be tumah from a corpse, meaning contact with a dead body; and even then, not every category of corpse tumah falls under this halachah, as the mishnah will explain.
"Al eilu tumos hanazir megale'ach" - these are the forms of tumah for which a nazir shaves:
"Al hameis" - a corpse. The Gemara adds that this includes a fetus, in a case where the entire body of the fetus is what conveys the tumah. Such a fetus does not convey tumah under the law of a limb severed from a corpse, since a limb is considered a limb only when it contains sinews, bone and flesh joined together, and in a fetus not all the sinews have yet developed. Its tumah stems solely from its being a complete body, even though it is an exceedingly small one.
"V'al kazayis min hameis" - an olive's measure of flesh from the corpse.
"V'al kazayis netzel" - netzel is flesh of the corpse that has decomposed and turned liquid, and it too conveys tumah.
"V'al melo tarvad rakav" - a very large spoonful, the measure of two handfuls, of rakav: a corpse that decomposed under extremely dry conditions until it turned to dust.
"Al hashidrah v'al hagulgoles" - a complete spinal column of bones, or a complete skull.
"V'al eiver min hameis" - a complete limb from the corpse, even if it does not have an olive's measure of flesh on it.
"V'al eiver min hachai she'yesh alav basar karaui" - even a limb severed from a living person, provided it has enough flesh on it that, had it remained attached, it could have healed and functioned again. It has the status of a corpse, and it is significant enough to convey tumah to the nazir.
"V'al chatzi kav atzamos v'al chatzi log dam" - these measures require special explanation.
Special measures introduced for the nazir:
The measure of a half-kav of bones is anomalous. In the laws of tumah generally, including tumas ohel - the stringent form of corpse tumah, in which being under one roof with the corpse renders a person tamei - bones convey tumah already at a quarter-kav. But for the nazir's obligation to shave there is a special halachah given to Moshe at Sinai, that this is relevant only with a half-kav. The measure is larger.
The same applies to the measure of blood: a half-log of blood from a corpse renders the nazir tamei, even though for corpse tumah in general a quarter-log suffices. Here too it is a special halachah from Sinai regarding the nazir, that he becomes tamei only with a half-log.
The modes of tumah - "al maga'an v'al masa'an v'al ohilan":
All these sources convey tumah to the nazir in any of three ways: if he touched them, if he moved them even without touching them, or if he was together with them under a single ohel, under the same roof.
"V'al etzem k'se'orah al maga'o v'al masa'o":
A bone the size of a barley grain conveys tumah to the nazir and obligates him to shave through touching and through carrying, but it does not convey tumas ohel, and therefore the law of ohel does not apply to it. Even with a bone this small, touching it or moving it conveys tumah.
The purification process and the offerings:
For all of these forms of tumah the nazir must shave his head, and be sprinkled with the waters of the parah adumah on the third day and on the seventh. And he cancels the days that preceded - the days he has counted until now are void, and he begins counting his new nezirus only after he becomes tahor: through the sprinkling of the parah adumah waters on the third day and on the seventh day, and immersion in a mikvah. The next day, on the eighth - after the seven day purification process - he brings the offerings of a tamei nazir, and only afterward does he begin the nezirus of his purity, his new term of nezirus.