Gemara

53b (Beraisa): "(Ba'Chalal) Cherev" teaches that a Cherev (sword) that killed a person is Tamei just like a Chalal (corpse).

54a (Mishnah): A Nazir does not shave for the following Tum'os:

Schachos or Pera'os (foliage of trees hanging over the ground, and rocks protruding from a wall); a field in which a grave was lost or plowed; Chutz La'aretz; Golel (a coffin cover or monument), Dofek (its supporting stones), a Revi'is of blood; a quarter Kav of bones; Kelim that touched a Mes, the days a Metzora counts between the two times he shaves...

We sprinkle (water with ashes of the red heifer) on him on days three and seven.

54b - Inference: Tum'ah was decreed on Chutz la'Aretz for towering above the soil. If it were only for entering the air, there would be no reason for Haza'ah (sprinkling)!

Rejection: The decree was for the air. Not all these Tum'os obligate Haza'ah.

Support: The Mishnah lists Kelim that touched a Mes, which do not obligate Haza'ah!

Tosefta (Makos 4:17): A Kohen is lashed mid'Oraisa for any Tum'ah for which a Nazir shaves (Gilu'ach Tum'ah).

Rishonim

Question (Tosfos Nazir 54b DH Ta): How did the Gemara prove from Kelim that Haza'ah does not apply to all of the Tum'os? Perhaps it refers to metal Kelim, which obligate Haza'ah!

Answer #1 (R. Tam): It cannot refer to metal Kelim. A Nazir must shave for them, for we expound that 'a sword (that killed someone) is like a corpse.'

Objection (R. Chaim Kohen): If so, Kohanim are commanded about this. It is impossible for a Kohen to be careful that his house not contain a nail or metal Kelim that was in Ohel ha'Mes! A Tosefta (Ohalos 1) says that Kelim Tamei through a Mes are Metamei people for seven days regarding Terumah and Kodshim, but a Nazir shaves only for a Mes, i.e. but not for them. The Tosefta discusses metal Kelim, for only they are Metamei for seven days. The Gemara did not say that we discuss metal Kelim, for in any case not everything obligates Haza'ah, e.g. days that a Metzora counts. Our Sugya refutes Rabbeinu Yitzchak me'Simpont, who says that any Keli that touched a Mes is like a corpse!

Defense #1 (of R. Yitzchak - Tosfos): The Gemara said 'Kelim do not obligate Haza'ah', i.e. 'what follows Kelim in the Mishnah, i.e. days of a Metzora...

Defense #2 (Hagahos Ashri Bava Basra 2:14 DH u'Chli): R. Yitzchak says that any Keli that killed a person becomes like a Mes.

Rambam (Hilchos Tum'as Mes 5:3): If a Keli became Tamei by touching a Mes or being in Ohel ha'Mes, one who touches it is Tamei for seven days.

Rambam (Hilchos Nezirus 7:7): The above Tum'os make a Nazir Tamei Shivah and require Haza'ah, but he does not shave for the Tum'ah. The days of Tum'ah do not count towards Nezirus.

Rambam (8): If a Nazir touched Ohel ha'Mes or a Keli touching a Mes, he does not need Haza'ah on days three and seven. I say that this is a special law of Nazir, to enable his days of Tum'ah to count towards Nezirus. Anyone else requires Haza'ah.

Rebuttal (Ra'avad): This is totally wrong. The Mishnah in Ohalos says that one gets Tum'as Shivah only for touching a Keli touching a Mes at the time.

Kesef Mishneh: Indeed, the Rambam discusses one who touched a Keli touching a Mes! He explains that the Mishnah exempts only a Nazir from Haza'ah when he became Tamei through Kelim. However, it is astounding to say that a Nazir is different than other people!

Ramban (Bava Basra 20a DH mi'Kol): If a Nazir were different, the Gemara would need to cite a tradition from Sinai for this! The Gemara connotes that his days of Tum'ah do not count towards Nezirus. Bamidbar 19:16 says that Tum'os, including a sword, impart Tum'as Shivah. Verse 18 mentions Haza'ah due to the same Tum'os, but omits a sword. This shows that a sword does not obligate Haza'ah. The Yerushalmi requires Haza'ah only for Tum'os for which one is liable for Bi'as (entering the) Mikdash. It requires Haza'ah for touching Kelim that touched a Mes, like the simple reading of the Mishnah in Nazir.

Radvaz: The Rambam holds like R. Yitzchak. He said 'I say that...' because one could answer like the Ra'avad, that the Mishnah in Ohalos discusses Tum'ah b'Chiburin.

Note: The Rambam is not totally like R. Yitzchak. R. Yitzchak holds that metal Kelim are Metamei even a Nazir Tum'as Shivah, therefore he said that the Mishnah discusses Klei Shetef.

Rambam (Hilchos Evel 3:1): A Kohen is lashed for becoming Tamei through touching, Ohel or moving a Mes or Tum'os that separated from a Mes.

Rosh (Chulin 8:4): Even though Kohanim are commanded about Tum'as Mes, they cannot be careful about a sword, which is like a corpse. Also, they are not commanded about it, for a Nazir does not shave for it.

Poskim

Rema (YD 369:1): Some forbid a Kohen to become Tamei to a sword that became Tamei through a Mes, and some are lenient. The custom is to be lenient.

Beis Yosef (DH u'Mah she'Chosav v'Lo l'Golel): Even though Maseches Semachos says that a Kohen is not lashed for any Tum'ah for which a Nazir does not shave, he is forbidden.

Gra (2): R. Tam says that a Nazir shaves for Tum'ah of a sword, and Kohanim are warned. The other Poskim hold that a Nazir does not shave for it, and Kohanim are not warned. The Beis Yosef holds that there is an Isur.

Beis Hillel: It seems that we discuss only touching. However, the stringent opinion is R. Tam, and he forbids even Ohel!