Gemara

42b: If Reuven stuck a piece of Nevelah down Shimon's throat:

If it is Belu'ah (considered enveloped), it is not Metamei;

If it is Beis ha'Starim, it is Metamei Shimon through Masa (moving it).

43b (Mishnah): A one-day old can become Tamei Mes.

44a: "V'Al ha'Nefashos Asher Hayu Sham" refers to any souls.

70b - Question (Alexandriyim): Was the Shunamis' son (whom Elisha revived) Tamei (like a Mes)?

Answer (R. Yehoshua): The dead are Teme'im (like a corpse), but the living are not.

Question #3: When the dead are revived, will they need Haza'ah (sprinkling of Mei Chatas on them)?

Version #1 - Answer (R. Yehoshua): When they are revived, we will investigate this.

Version #2 - Answer (R. Yehoshua): When Moshe will come with them (he will tell us).

Nazir 43a (Beraisa): "(A Kohen is forbidden Lehechalo (to profane himself)" - he is not profaned until the time of death (when there is a Chalal, i.e. a corpse);

Rebbi says, "he (a Nazir) may not be Mitamei for them (his relatives) in their deaths" - the Isur is not until they die.

(R. Yochanan): They argue only about how we derive the Halachah.

(Reish Lakish): They argue about a Goses. The first Tana includes a Goses from Lehechalo, for he will become a Chalal. Rebbi disagrees, due to "in their deaths".

Chachamim hold that even though a Goses is not Metamei, a Kohen transgresses for him.

Rishonim

Rambam (Hilchos Evel 3:5): If Reuven was Metamei a Kohen, if both of them were Mezid, the Kohen is lashed and Reuven transgresses "v'Lifnei Iver Lo Siten Michshol." If the Kohen was Shogeg and Reuven was Mezid, Reuven is lashed.

Rambam (Hilchos Tum'as Mes 3:1): Ever Min ha'Mes or Ever Min ha'Chai that has enough flesh (to sustain it) is Metamei b'Ohel (everything under the same roof as it), Maga (touching) and Masa.

Rambam (Hilchos Tum'as Mes 2): If Ever Min ha'Chai does not have enough flesh, it is Metamei b'Maga and b'Masa, but not b'Ohel.

Rambam (25:11): If a woman gave birth to twins, and the first came out dead and the latter lived, he (the latter) is Tahor, for he did not touch the dead baby from when he was born.

Rebuttal (Ra'avad): The latter baby cannot be Tahor, for his mother is Tamei (due to the dead baby), so she is Metamei the latter baby Tum'as Erev (until he is immersed) when he is born (and touches her)!

Perush ha'Rosh (Nazir 43a DH l'Inyan): (Reish Lakish explains that) even though a Goses is not Metamei until he dies and "Lo Yetamei" does not apply, a Kohen is lashed for Lehechalo. Since Lehechalo is written near Lo Yetamei, it is as if it says Lo Yetamei v'Lo Lehechalo. The Halachah follows R. Yochanan against Reish Lakish. The Tana'im argue only about how to expound. Therefore, a Kohen is not commanded about a Goses. However, in Bahag's text, Abaye and Rabah argue about how to explain the Beraisa. The Halachah follows Rabah, who says that they argue about a Goses, and we rule like Chachamim against Rebbi, so a Kohen is commanded about a Goses.

Hagahah (2): Our text of Bahag says that the Halachah follows Rebbi, for the Stam Mishnah is like him. (Perhaps the Rosh means that according to Bahag's text, the Halachah follows Chachamim, and a Kohen is commanded. The Rosh does not learn from the Stam Mishnah like Bahag does - PF.)

Tosfos (70b DH Mesim): Why didn't they ask whether the Shunamis' son needed Haza'ah? Perhaps they asked whether "v'Zarakti Aleichem Mayim Tehorim" refers to sprinkling on days three and seven, but surely the Shunamis' son did not need Haza'ah.

Maharsha (69b Chidushei Agados): They did not ask whether the Mesim who are revived will be Metamei, for the old body decayed and they are new bodies. They asked whether one who was Tamei Mes when he died will need Haza'ah after he is revived. They did not ask about the Shunamis' son, for he had the same body. If he was Tamei, his Tum'ah would not go away without Haza'ah, more than if he never died. They thought that also the Neshamah becomes Tamei Mes. They erred. It is spiritual. It does not become Tamei at all.

Poskim

Shulchan Aruch (YD 370:1): A Goses is not Metamei until he dies. In any case, (a Kohen) may not enter a room in which there is a Goses. (Some permit.)

Shach (YD 371:1): The Roke'ach (366) permits a pregnant Kohen's wife to enter Ohel ha'Mes, for it is a Sefek-Sefeka. Perhaps the fetus is a female, and even if it is a male, perhaps it is a Nefel.

Question (Magen Avraham 343:2): This is difficult. Even without this it is permitted, for Tum'ah Belu'ah is not Metamei (and Taharah Belu'ah does not become Tamei - PF)! The Rambam and Ra'avad (Hilchos Tum'as Mes 25:1) say so. (The Ra'avad agrees that the live baby does not become Tamei before it is born.)

Pischei Teshuvah (371:1): The Radvaz (200) answers that the Roke'ach permits a woman due to give birth. If he stuck his head outside, this is considered birth, and this is not Taharah Belu'ah. The Roke'ach teaches that we are not concerned for this, so a Sefek-Sefeka permits.

Achi'ezer (3:65): The Roke'ach holds like Bahag. There is an Isur to enter Ohel ha'Mes even if he does not become Tamei. Therefore, he permits only due to a Sefek-Sefeka. The Minchas Chinuch (298) held that obviously, a Kohen does not transgress for Safek Tum'ah in Reshus ha'Rabim, since it is Tahor. I say that this is not obvious. Perhaps it is a Safek Isur, even though he remains Tahor, like one who enters a room with a Goses!

Kovetz Shi'urim 2:41: Perhaps the Roke'ach holds like those who forbid a Kohen to enter Ohel ha'Goses, even though he does not become Tamei. Taharah Belu'ah is considered to be in Ohel ha'Mes, just it is a Gezeras ha'Kasuv that it remains Tahor. Therefore, this does not permit a Kohen.

Arugas ha'Bosem (YD 248): Even if the Goses recovered and did not die, the Kohen transgressed for entering the room with him.

Chelkas Yo'av (Kava d'Kashyasa 43): Why is the Roke'ach concerned for a fetus? A verse taught that a one-day old can become Tamei Mes. This implies that a fetus cannot become Tamei Mes! If a pregnant woman became Tamei Mes, we need not sprinkle on the child after he is born (Parah 3:2).

Yachel Yisrael (75): The Rambam did not mention entering a room with a Goses. He holds that there is no Lav for this. Chelkas Yo'av (2:9) argues with Achiezer and Kovetz Shi'urim. He holds that there is no Isur when he does not become Tamei.

Yachel Yisrael (75): A Kohen may not become Tamei Mes, even through Ever Min ha'Chai. This is a Torah Lav. Also Yisre'elim may not be Metamei him. May a Kohen receive a transplant? If it is Piku'ach Nefesh and (mortal) danger, surely all Aveiros (with three exceptions) are permitted to save him, and others may transgress to save him. Is it permitted even if it is not Piku'ach Nefesh? The limb that will be transplanted is Metamei the room. It is possible to get limbs from a Nochri. They do not have Tum'as Ohel, only Tum'as Maga and Masa. We discuss Tum'as Maga and Masa. If it is an internal limb, Tum'ah Belu'ah is not Metamei b'Maga, but it is Metamei b'Masa. If it is an external limb, there is Tum'as Maga and Masa. However, even though Tum'ah Belu'ah is not Metamei, perhaps it is Asur for a Kohen to enter Tum'ah into himself.

Suggestion: This is unlike entering a room with a Goses. There, he may die at any moment. Here, the limb is Belu'ah, so there is no Tum'ah at all! Also, Lehechalo applies to entering an Ohel. It does not apply here, for there is no Tum'as Ohel!

Rejection (Yachel Yisrael): Since the Torah forbids even when there is no Tum'ah, perhaps the same applies to Tum'ah Belu'ah. However, regarding a transplant, there is no Ever Min ha'Mes. Toras Micha'el and ha'Gaon R. A.Y. Unterman Ztz"l said that a transplant of a cornea from a Mes' eye does not entail Isur Hana'ah from a Mes, since it becomes part of the living person. The Isur Hana'ah goes away. Likewise, in a transplant the Tum'ah goes away. Therefore, a Kohen may receive a transplant even if it is not Belulah and not Beis ha'Setarim.

Yachel Yisrael: In Ohalos (2:2), Tana'im argue about a k'Zayis of worms from a Mes, whether they are alive or dead. Mishnah Acharonah explains that R. Eliezer is Metamei because it is like the Mes' flesh. Even if the worms are alive, they are not considered alive. Chachamim say that when they are alive, this uproots the Tum'ah. R. Eliezer argues only because he does not consider them alive regarding Tum'ah. Both agree that if Tum'as Mes returns to live, there is no Tum'ah. Ha'Gaon R. Y. M. Erenberg, Ztz"l brought this proof. Shevet me'Yehudah (1 p.59) rejected the proof, and cited Tif'eres Yisrael in Ohalos (2:22, who says that Chachamim are Metaher because the worms are a new creation).

Yachel Yisrael: Rashi explains that they asked whether the Shunamis' son was considered dead. R. Yehoshua answered that he returned to life. This shows that if a Mes returns to live the Tum'ah goes away. Likewise, the Isur Hana'ah goes away. This is why Toras Micha'el and R. Unterman permitted the Karnis transplant. Seridei Esh questioned this, for even though the flesh returns to life, he benefits from the Mes. This depends on how we understand the Isur Hana'ah of a Mes. However, there is no reason to say that the Tum'ah remains. Also, we can join the opinion of the Shach (YD 157:3) who was unsure whether we permit Isurei Torah when there is danger to a limb. He was lenient for a Lav, but not for Shabbos.

Igros Moshe (YD 1:230, Anaf 3, and b'Sof): The Yere'im and others hold that a Kohen is not commanded about Tum'as Mes of Nochrim. If a Kohen needs a transplant (in his leg), if it will be totally covered (with his own skin), it is totally Belu'ah. Even if a little skin of the Mes will be revealed externally, it is Batel to his body, and becomes Tahor. However, they must tie his leg so he cannot move it until the skin is sewn up over the transplant. If it is possible that the doctors will not move his leg until then, they must do so. If they cannot, we can permit if it is flesh of a dead Nochri, for there is a Sefek-Sefeka (Even if the Halachah does not follow the Yere'im, some hold that he does not become Tamei if others move his leg.)