Answer: Tum'ah is different, because it says "he defiled Rosh Nizro." This refers to one whose Nezirus depends on his head (i.e. until he shaves).

Question (Beraisa): A Nazir who finished his term may not shave, drink wine, or become Tamei. If he did any of these, he receives 40 lashes. (The Mitzvah to shave is only after bringing (at least one of) the Korbanos.)

This refutes R. Yosi b'Rebbi Chanina.

(Mishnah): If one said 'I will be a Nazir when I have a son. I am a Nazir 100 days', and he had a son within 70 days of saying this, he does not lose any of the days counted. (The subsequent days count for both. He shaves once after finishing the long Nezirus.)

If he had a son after 70 days, he loses (70 - Tosfos 13b DH l'Acher deletes this from the text), since shaving requires 30 days growth of hair.

(Gemara - Rav): (If a son was born on) the 70th (or any previous) day, the day counts for both Neziruyos.

Question (Mishnah): If he had a son within 70 days of saying this, he does not lose any of the days counted.

If the 70th day counts for both, why does it say 'he does not lose'? He profits! (Mid'Oraisa, Miktzas ha'Yom k'Kulo, i.e. part of the 100th day is like the full day. Mid'Rabanan, he may not shave until day 101. However, if a son was born on day 70, he may shave on day 100, for it is day 31 of his Nezirus!)

Answer: Indeed, the Reisha should not have said 'he does not lose' (rather, 'he profits.') 'until It says so for parallel structure with the Seifa, which says 'after 70, he loses.'

Question (Mishnah): If a son is born after 70 days, he loses (we do not say that day 71 counts for both!)

Answer: 'After 70' means 'after the day after 70', i.e. 72.

Objection: If on day 71 he does not lose, the Reisha should say 'until 71 days, he does not lose!'

Conclusion: 'After 70' really means '71'. The Mishnah opposes Rav.

Question: Rav must hold like another Tana. Likewise whom does he hold?

Answer #1: Rav holds like Aba Sha'ul:

(Beraisa): If a mourner buried his relative three days before Yom Tov, Yom Tov cancels Shivah (the restrictions of the first seven days of mourning);

If burial was eight days before Yom Tov, Yom Tov cancels Sheloshim (the restrictions of the first 30 days), and he may shave on Erev Yom Tov. If he did not shave on Erev Yom Tov, he may not shave after Yom Tov (for the rest of the 30 days);

Aba Sha'ul says, even if he did not shave on Erev Yom Tov, he may shave after Yom Tov. Just like fulfilling three days of mourning nullifies the rest of Shivah, also seven days nullifies the rest of Sheloshim.

Question: What is Aba Shaul's reason?

Answer: The seventh day counts towards Shivah, and also towards Sheloshim.

Objection: Perhaps Aba Sha'ul says so only about Shivah, which is mid'Rabanan, but not about Nezirus, which is mid'Oraisa!

Answer #2: Rav holds like R. Yosi:

(Mishnah - R. Yosi): The Korban Pesach was slaughtered for (a group including) a Shomeres Yom k'Neged Yom (a woman on the morrow of seeing non-menstrual blood). Later that day, she saw blood. She cannot eat the Pesach at night, but she is exempt from Pesach Sheni (the compensatory Korban for one who did not fulfill Korban Pesach).

Question: What is R. Yosi's reason? (We explain according to Hagahos ha'Gra.)

Suggestion: He holds that Miktzas ha'Yom k'Kulo. (Since she did not see blood at the beginning of the day after her first sighting, it is like a full clean day.) Therefore her next sighting does not (join with the first to) make her Teme'ah (a Zavah Ketanah) retroactively. (She becomes Teme'ah only from when she saw blood. Since she was Tehorah at the time of Shechitah, she was Yotzei (fulfilled her obligation).)

Question: R. Yosi holds that she is Teme'ah retroactively!

(Beraisa - R. Yosi): The Korban Pesach was slaughtered for a Zav who had two emissions on the seventh day (from his last emission) and a Shomeres Yom k'Neged Yom. Later, each had another sighting. Even though retroactively they are Metamei Mishkav u'Moshav (if either sat on a Kli made for sitting, it becomes an Av ha'Tum'ah), they are exempt from Pesach Sheni.

Answer: The retroactive Tum'ah is mid'Rabanan. (Mid'Oraisa, they are Tamei only from when they see.)

Support: This must be so. If not, they would be obligated to bring Pesach Sheni!

Rejection: Perhaps the retroactive Tum'ah is mid'Oraisa. Tum'as ha'Tehom (a Tum'ah that no one knew about it) of emissions does not obligate a person to bring Pesach Sheni.

R. Oshaya also holds that there is retroactive Tum'ah mid'Rabanan:

(Beraisa - R. Oshaya): (A Zav must count seven days without emissions to become Tahor.) If a Zav had an emission on the seventh day, this cancels the days he counted.

R. Yochanan: It should cancel only the seventh day!

Objection: This is illogical! If it cancels the day, it should cancel the entire count. If it does not cancel the day, the count should be complete!

Correction: Rather, R. Yochanan says that even the seventh day counts.