Gemara

Rabah bar Avuha (to Eliyahu): Aren't you a Kohen (Pinchas)? Why are you in a cemetery (of Nochrim)?!

(Eliyahu - Beraisa - R. Shimon): Graves of Nochrim do not impart Tum'as Ohel.

Nazir 48a (Beraisa) Suggestion: Perhaps "...Al Nefesh Mes Lo Yavo" means even an animal, which is called "Nefesh Behemah"!

Rejection #1: "Al Nefesh Mes" refers to a person.

Rejection #2 (R. Yishmael): We need not learn from there. "Lo Yavo" discusses a Tum'ah imparted by entering (an Ohel).

64b (Mishnah): If one finds a Mes lying down, like normal, he may relocate it, taking its Tefusah (underlying soil) with it. The same applies if he finds two Mesim. If he finds three Mesim between four and eight Amos apart, the area is assumed to be a burial site. He must check 20 Amos for more Mesim.

(Rav Yehudah): The Mishnah says 'if he found', to exclude a known Mes. 'Dead' excludes someone murdered. 'Lying' excludes sitting. 'Normally' excludes when the head rests between the thighs.

(Ula bar Chanina): An incomplete Mes does not have the law of Tefusah, and it does not count towards establishing a burial site.

All of these cases are excluded, for perhaps the Mes is a Nochri.

Yevamos 61 (Beraisa - R. Shimon): Tum'as Ohel (being Metamei everything under the same roof as the Tum'ah) does not apply to graves of Nochrim. Regarding Tum'as Ohel it says "Adam Ki Yamus b'Ohel", and) it says "you are My flock... you are Adam." You (Yisrael) are called Adam, but Nochrim are not.

Questions: "V'Nefesh Adam Shishah Asar Elef" (the captives taken from Midyan are called Adam)! "(Ninveh) has more than 120,000 Adam..."!

Answer: It says Adam to distinguish the people from the animals.

Question: "Whoever killed a soul (in the war against Midyan) and touched a Mes will make himself Tahor" (shows that a dead Nochri is Metamei)!

Answer #1: Perhaps a Yisrael died.

Chachamim do not accept this answer, for it says "v'Lo Nifkad Mimenu Ish";

R. Shimon explains that verse to mean that no one sinned.

Answer #2 (Ravina): "Adam Ki Yamus b'Ohel" excludes only Tum'as Ohel. We do not exclude Tum'ah of carrying or touching!

Rishonim

Rambam (Hilchos Evel 3:3): A Nochri is not Metamei b'Ohel. Therefore their graves are Tehorim. A Kohen may enter there and trample on their graves. It is forbidden only to touch or carry Tum'ah (of a dead Nochri).

Radvaz: Even though R. Shimon ben Gamliel holds like Chachamim (Ohalos 18:9), and this is not one of the three places in which the Halachah does not follow R. Shimon ben Gamliel in the Mishnah, the Rambam rules like R. Shimon due to the episode with Eliyahu. The Rambam holds that Eliyahu gave a proper answer; also, Eliyahu relied on it in practice.

Hagahos Maimoniyos (2): The Re'em rules like R. Shimon, that Nochrim do not have even Tum'as Maga or Masa. R. Tam rules like Chachamim, because R. Shimon ben Gamliel holds like them. Eliyahu gave a mere Dichuy. He went over Nochri graves because there was a gap of a Tefach above the Mes (so the Tum'ah stays inside). Seder Eliyahu Rabah says that he descended from Rachel (so he is not a Kohen).

Question (Mishneh l'Melech): Chachamim forbade Kohanim to go to Chutz la'Aretz (Avodah Zarah 13b). They declared Chutz la'Aretz to be Tamei because Nochrim bury their dead anywhere. According to the Re'em, there is no Tum'ah at all! It is difficult to say only Chachamim say that there was a decree. Perhaps the Re'em holds like Tosfos (Nazir 54b DH Eretz), who says that it was due to the many Yisraelim killed in Chutz la'Aretz.

Rambam (Hilchos Tum'as Mes 1:13): A Nochri is not Metamei b'Ohel. This is a tradition. Regarding the war with Midyan it says 'anyone who touched a corpse'. It does not mention an Ohel.

Kesef Mishneh: The Rambam rules like R. Shimon against Chachamim because it seems that Ravina holds like R. Shimon. Also, Eliyahu said that this is the Halachah.

Question: Lo ba'Shamayim Hi! (Bava Metzia 59b teaches that Nevi'im cannot tell us what is the Halachah.)

Answer (Birkei Yosef OC 32:4 DH v'Nir'eh): They cannot tell us whom the Halachah follows through prophecy, but a Navi can say things from his Chachmah. When the Gemara says 'if Eliyahu will come and say... we will not heed him', it means 'if he will say through prophecy.'

Rambam (9:4): A Nochri's coffin is not Tamei, since a Nochri has no Tum'as Ohel. One becomes Tamei only through touching or moving the Tum'ah itself.

Kesef Mishneh: He learns from Nazir 65a (Nochrim do not count towards establishing a burial site).

Note: Perhaps it is obvious that Nochrim have cemeteries, so if they have Tum'as Ohel, there would be no reason not to be concerned.

Mishneh l'Melech (Hilchos Beis ha'Bechirah 1:13): The Yerushalmi (Sanhedrin 1:2) says that Chizkiyah was Me'aber the year because the skull of Arona ha'Yevusi was found under the Mizbe'ach. Rashi (Sanhedrin 12a DH she'Iber) says that it was due to Tum'as Avodah Zarah the days of Achaz. Rashi could hold that the Halachah follows Chachamim, just he seeks to explain also according to R. Shimon.

V'Shav ha'Kohen (75, cited by R. Akiva Eiger): Even according to the opinion that a Ger Toshav must accept all the Mitzvos except for eating Neveilos, R. Shimon holds that he is not called "Adam" and has no Tum'as Ohel. We always follow Tosfos against the Rambam, therefore one must be stringent.

Teshuvas ha'Rambam (145 DH ha'Teshuvah): Clearly there is no difference between touching a dead Nochri, or any Mes, and his Kever. It says "V'Kol Asher Yiga ...ba'Chalal ...Oh v'Kaver", and regarding the war with Midyan it says "V'Kol Noge'a be'Chalal". Just like one who touches a Nochri is Tamei, also one who touches his Kever. Both have Tum'as Maga, but not Ohel. Ravina (Yevamos 61a) says so according to everyone. Had he answered only for R. Shimon, he would have said 'R. Shimon can answer...' Eliyahu towered about Nochri graves, he did not touch them. R. Efrayim says that the stringency of Nochri residences shows that Nochrim have Tum'as Ohel. This is wrong. It is a stringency mid'Rabanan. This is why one need not excavate all the ground underneath.

Note: It is not clear how to resolve this with what the Rambam says in the Mishnah Torah.

Teshuvas ha'Rosh (30:1): Rabanan argue about Tum'as Ohel of Nochrim. Maharam me'Rotenburg protested against Kohanim who walked over Nochri graves. One who is stringent will be blessed.

Panim Me'iros (2:14, b'Sof): In Avodah Zarah 39a, the Gemara rules about Kever fish. Rashi's text says 'Kever-fish is forbidden. A Siman (way to remember this) is Kivrei (graves of) Nochrim (they are Tamei).' This is difficult. If the Gemara holds that Kivrei Nochrim are like Kivrei Yisrael, it should have simply said that Kever is the Siman! Rather, we must adopt the text of R. Chananel (brought in Tosfos DH v'Simanech), that Kever-fish is permitted, and the Siman is Kivrei Nochrim (which are Tahor)! This shows that the Halachah follows R. Shimon.

Poskim

Shulchan Aruch (YD 372:2): It is proper that a Kohen be careful not to walk over graves of Nochrim.

Note: After the Beis Yosef (DH Kivrei) brings the opinions of the Rambam, R. Tam and the Rosh, and the Re'em, he says 'l'Halachah we are stringent.' The Shach explains that he rules like R. Shimon, and is stringent like the Rambam against the Re'em. The next laws in the Beis Yosef are like R. Shimon. However, the Mishneh l'Melech (Evel 3:3) says that the Beis Yosef is stringent like R. Tam, and so he rules in the Shulchan Aruch.

Rema: Even though some are lenient, it is proper to be stringent.