Gemara

(Beraisa): If one went on a road and underneath was found a Mes spanning the width, he is Tamei regarding Terumah, but Tahor regarding the Korban Pesach or Nezirus. Tum'as ha'Tehom applies only to a Mes.

Pesachim 80b - Question: What does this exclude?

Answer #1A: It excludes Tum'as Sheretz of the (Korban's) owner. Only Tum'as Mes interrupts Nezirus. Therefore, it must discuss Pesach.

Question: According to the opinion that we offer Pesach even for one who is known to be Tamei Sheretz, how can we answer?

Answer #1B: It discusses Tum'ah of the Kohen (offering the Korban).

Answer #2 (Rav Yosef): The Beraisa discusses Tum'ah of the owner regarding Pesach. It exclude Tum'as ha'Tehom of Zivah (retroactive Tum'ah due to a later sighting of blood or Zov. At the time, no one knew about it). He was not Yotzei.

81a (Rav Yosef) Question: Is Tum'as ha'Tehom permitted for a Kohen offering the Tamid? If you will say that it is permitted to a Kohen offering Korbanos (Nazir or Pesach), perhaps the tradition is only for these. Or, do we learn to the Tamid?

Nazir 15b - Question: R. Yosi holds that Zivah is Metamei retroactively!

(Beraisa - R. Yosi): The Korban Pesach was slaughtered for a Zav who had two emissions on the seventh day (from his last emission). Later, he had another sighting. Even though retroactively he is Metamei Mishkav u'Moshav (a Keli made for sitting), he is exempt from Pesach Sheni.

Answer: The retroactive Tum'ah is mid'Rabanan. (Mid'Oraisa, he is Tamei only from the next sighting.)

Support: This must be so. If not, he would be exempt from Pesach Sheni!

Rejection: Perhaps the retroactive Tum'ah is mid'Oraisa. Tum'as ha'Tehom of Zivah does not obligate a person to bring Pesach Sheni.

65a (Rav Yehudah): The Mishnah says 'Mes' to exclude someone murdered. (He does not join to establish a burial site.)

Rishonim

Rambam (Hilchos Korban Pesach 6:3): If a Zav had two emissions, counted seven clean days, immersed, and the Korban Pesach was slaughtered for him, and he had another emission after Zerikah, he is exempt from Pesach Sheni.

Rambam (Hilchos Nezirus 6:18): Tum'as ha'Tehom applies only to one who died, but not to one who was killed, for the murderer knew.

Rebuttal (Ra'avad): No, 'it applies only to a Mes' excludes Zivah.

Defense (Radvaz): Also the Rambam excludes Zivah. (The Reisha, 'it is a Tum'ah that no one in the world knew about it', excludes one murdered.)

Question (Ri Korkus and Lechem Mishneh Nezirus 4:4): Regarding Pesach the Rambam rules like R. Yosi, who says that mid'Oraisa the Tum'ah is not retroactive. In Hilchos Isurei Bi'ah (6:15), he rules like Rabanan, who say that the Tum'ah is retroactive mid'Oraisa!

Answer #1A - Lechem Mishneh: Nazir 15b permits Tum'as ha'Tehom of Zivah! The Ra'avad must say that the Tana in Pesachim disagrees.

Rebuttal (Shirei Korban Nazir 44b DH Ha): In Pesachim we conclude that Tum'as ha'Tehom of Zivah was not permitted! Nazir 15b is a mere Dichuy.

Answer #1B (Orach Mishor Nazir 15a DH Kosav): The Rambam always rules like 'if you will say that...' In Pesachim, we asked 'if you will say that Tum'as ha'Tehom is permitted to a Kohen offering...' This shows that this is the Halachah. Therefore, 'only for Mes' does not exclude Zivah, rather, Sheretz.

Rebuttal (Shirei Korban, ibid.): If we are lenient about Tum'as ha'Tehom for a severe Tum'ah (Zivah), all the more so for a light Tum'ah (Sheretz)! Also, the Rambam does not mention Tum'as ha'Tehom of Sheretz!

Defense (Keren Orah Nazir 16a DH v'Achar): The Yerushalmi makes such a Kal va'Chomer. Perhaps the Bavli does not. The Rambam permits Tum'as ha'Tehom of all Tum'os, including Sheretz. 'Mes' excludes only someone murdered. He exempts from Pesach Sheni not due to Tum'as ha'Tehom, rather, because retroactive Tum'ah is not strong enough to disqualify Pesach.

Note: Three great Amora'im of Eretz Yisrael learned Tum'as ha'Tehom from verses (Pesachim 81a). The Bavli concluded that it is a tradition from Sinai. We do not make a Kal va'Chomer from a tradition from Sinai (Nazir 57a).

Answer #1C (Tzelach Pesachim 81a DH Shuv): The beginning of the Sugya learned from verses that Tum'as ha'Tehom is one that was not known to the one who became Tamei, or two people, or everyone. This forced us to say that 'Mes' excludes Zivah or Sheretz. We conclude that Tum'as ha'Tehom is not known to anyone, so 'Mes' excludes one murdered.

Rebuttal #1 (of all answers - Sefas Emes Nazir 15b DH Gam): Perush ha'Mishnayos (Pesachim 80b) permits only Tum'as ha'Tehom of a Mes.

Rebuttal #2 (Chazon Ish OC 124 for Daf 81a): If Tum'as ha'Tehom of Zivah were permitted, why would Chachamim obligate Pesach Sheni? (The Gemara never said that the Tana'im argue about Tum'as ha'Tehom of Zivah.)

Answer #2 (Shirei Korban, ibid.): Rather, the Rambam learned from Rav Yehudah (65a), who says that 'Mes' excludes someone murdered. The Gemara in Pesachim answered on behalf of Rava, who disagrees.

Answer #3 (Chidushei R. Meir Simchah Nazir 63 [5]): The Rambam says that 'Mes' exclude someone murdered, but he cannot permit Tum'as ha'Tehom of Zivah, unlike many Gemaros! Rather, he rules that mid'Oraisa, even a Zav may send his Korbanos. Regarding Pesach, he must be able to eat at night. Since he immersed before Zerikah, he was destined to eat at night. If he happened to become Tamei after Zerikah, even if Tum'ah is retroactive, (regarding ability to eat at night) it is as if he was Tahor until after Zerikah. The Sugya in Pesachim is like Rav, who holds that mid'Oraisa we do not offer Pesach for a Tamei Sheretz. One who did not immerse cannot be Yotzei. Since the Tum'ah is retroactive, it is as if he did not immerse.

Retraction (and Answer #4 - Or Some'ach): This is difficult. Rather, Tum'as ha'Tehom of Zivah is totally retroactive if he did not immerse. He was always Tamei, so it is not considered Tum'as ha'Tehom. The Rambam permits only if the Zav already immersed. Then, the Tum'ah is not retroactive regarding Klei Cheres that he moved). Therefore, the law of Tum'as ha'Tehom applies.