1) THE "TUM'OS" OF "SECHACHOS" AND "PERA'OS"
OPINIONS: The Mishnah states that a Nazir does not shave for the Tum'ah of "Sechachos" or for the Tum'ah of "Pera'os." The Mishnah in Ohalos (8:2) explains that "Sechachos" are branches of trees, and "Pera'os" are stones that protrude from a fence. What Tum'ah do these items have, and why does a Nazir not shave for them?
(a) TOSFOS (54b, DH u'Pera'os) and the ROSH explain that the Mishnah refers to a case in which a corpse is buried underneath one of the branches of a tree or protruding stones of a fence, but the exact location of the grave is not known. In such a case, all of the protruding branches or stones are Metamei out of doubt. Since the Tum'ah is only a Safek Tum'ah, the Nazir does not shave.
Why is the Nazir not required to shave out of doubt in order to complete his Nezirus Taharah? Tosfos in Nidah (57a, DH Ilan) and the Rosh here explain that the branches or stones protrude into Reshus ha'Rabim. There is a rule that any Safek Tum'ah in Reshus ha'Rabim is deemed to be Tahor (Sotah 28a). Since, mid'Oraisa, a Safek Tum'ah in Reshus ha'Rabim is Tahor, the Tum'ah caused by Sechachos and Pera'os is only mid'Rabanan. (Perhaps the Rabanan were stringent in this particular case of Safek Tum'ah in Reshus ha'Rabim because it is more common than other forms of Safek Tum'ah.) Therefore, mid'Oraisa the Nazir is not Tamei and has no obligation to shave.
(b) TOSFOS in Nidah (57a) gives a second explanation for the Tum'ah of Sechachos and Pera'os. Sechachos and Pera'os are branches and stones which are each less than a Tefach in width. Even though a Mes is buried in the ground beneath them, mid'Oraisa they do not transfer the Tum'ah to any object beneath another area of the branch. The Rabanan, however, decreed that since the branches or stones are so close together, they must be viewed as though they are joined and are a Tefach wide, and thus they are Metamei the objects beneath them.
This is also the explanation of Tosfos here (DH Ilan). Apparently, the words of Tosfos here are the words of a Hagahah who argues with the following Tosfos, who gives the other explanation of Sechachos and Pera'os (see Arzei ha'Levanon in the name of the Mei Nafto'ach who attempts to reconcile the two explanations of Tosfos).
Why, though, did the Rabanan decree that the branches are Metamei in this case if they are actually separate from each other? Normally, even an extremely thin breach between two parts of a roof effectively splits the two sides into two separate roofs and annuls the Ohel!
1. The RASH (Ohalos 8:2) suggests that since the branches are close enough that they would support a thin layer of cement poured upon them to form a platform, the Rabanan decreed that they are Metamei through Ohel, as the Mishnah there says. The other Mishnayos which imply that a split of less than a Tefach splits the Ohel into two refer to a split which cannot support even a thin layer of cement.
2. The RAMBAN writes that the Rabanan were stringent here because all of the branches or stones come from a single source; they are all connected at one end. Therefore, the branches are viewed at the unconnected end as if they are connected there as well.
3. TOSFOS in Nidah (57a) and the RASH in his second explanation explain that there is no space between the branches. Why, then, does a Nazir not shave for the Tum'ah beneath them? Tosfos writes that although there is no space between the branches horizontally, there is space between them vertically. The branches are not on one level; some are higher and some are lower. Although the principle "Chavot Rami" (see Sukah 18a) teaches that the higher branches should be viewed as though they are on the same level as the lower branches, apparently the branches are arranged in such a way that "Chavot Rami" does not apply (for example, they are more than three Tefachim away from each other vertically).
The Rash explains that even if all of the branches are on the same level, they are Metamei only mid'Rabanan since they blow around in the wind and separate from each other.

54b----------------------------------------54b

2) A NAZIR WHO BECOMES "TAMEI" WITH "CHEREV K'CHALAL"
OPINIONS: The Gemara proves that Haza'ah for the Nazir is not necessary in all of the cases of Tum'ah listed in the Mishnah (54a). It proves this from the part of the Mishnah which mentions "Kelim ha'Nog'im b'Mes," utensils that came into contact with a corpse. Since a Nazir who touches such utensils does not need Haza'ah, in other cases of the Mishnah as well the Nazir does not need Haza'ah.
Why does the Gemara assume that a Nazir who touches such utensils does not need Haza'ah? The Torah teaches that "Cherev Harei Hu k'Chalal": a utensil which comes into contact with a corpse acquires the same level of Tum'ah as the corpse (Avi Avos ha'Tum'ah). Similarly, a utensil which comes into contact with a Tamei object received its Tum'ah from a corpse has the same degree of Tum'ah as that object (Av ha'Tum'ah). (The Rishonim disagree about whether this rule applies only to metal utensils or to all utensils other than earthenware. See Insights to Pesachim 14:2.) According to this rule, the Nazir who touched a utensil which touched a corpse has the same Tum'ah as one who touched the corpse itself, and thus he should require Haza'ah. (TOSFOS DH Ta Shema)
In order to answer this question, further elucidation of the Halachah of "Cherev Harei Hu k'Chalal" is necessary. In what way is the utensil which touches a Mes like a Mes? To what degree does the comparison extend?
There are a number of different opinions in the Rishonim.
(a) RABEINU TAM (cited by Tosfos here) rules that the Cherev (sword, or utensil which touched the Mes) is like the Mes itself in all ways. Not only does a person who touches the Cherev become an Av ha'Tum'ah and needs Haza'ah, if a Nazir touched it he must shave as if he touched a Mes. Therefore, when the Mishnah says that a Nazir does not shave when he touches utensils which touched a Mes, it refers to non-metal utensils which are not "k'Chalal," like the Mes. When a Nazir touches such utensils he becomes Tamei only with Rishon l'Tum'ah and does not need Haza'ah (rather, he becomes Tahor at the onset of evening).
This is a novel explanation. The reason why the Tum'os listed in the Mishnah do not require a Nazir to shave is that these Tum'os are not mentioned explicitly in the verse but are derived through Divrei Kabalah, through a Halachah l'Moshe mi'Sinai. The rule is that a Nazir shaves only for a Tum'ah of a Mes which is written explicitly in the Torah. Since the Tum'ah of "Cherev k'Chalal" is not written explicitly in the Torah, why does the Halachah l'Moshe mi'Sinai require the Nazir to shave for such a Tum'ah? Rabeinu Tam apparently maintains that this is part of the Halachah l'Moshe mi'Sinai; the Halachah states that this form of Tum'ah should be treated as though it were written explicitly in the Torah.
(b) Most Rishonim maintain that a Nazir does not shave for the Tum'ah of "Cherev k'Chalal" (see Tosfos here in the name of Rabeinu Chaim Kohen). According to these Rishonim, why does the Gemara assume that Haza'ah is not necessary for such a Tum'ah? The Rishonim explain that the Gemara's proof is not actually from the case of utensils which touched a Mes, but from the case of Tum'as Metzora (which is also listed in the Mishnah, and for which a Nazi obviously does not need Haza'ah since Haza'ah is necessary only for Tum'as Mes). (The Tosfos Rid actually changes the Girsa in the Gemara so that it explicitly states that the proof is from the case of Metzora.)
(c) According to other Rishonim, a Cherev is Metamei only through Maga and Masa but not through Ohel, even though it is an Avi Avos ha'Tum'ah like the Mes itself (RAMBAN to Bava Basra 20b and Bamidbar 19:14; see also TESHUVOS HA'RASHBA 1:476, and TOSFOS to Chulin 72a). They explain that although the verse which teaches "Cherev Harei Hu k'Chalal" (53b) discusses Tum'as Ohel, nevertheless another verse (Bamidbar 19:14) excludes everything except for a Mes from being Metamei through Ohel. Only the actual body of a deceased person is Metamei through Ohel.
This answers the question of RABEINU CHAIM KOHEN cited by Tosfos here. Rabeinu Chaim Kohen asks that if a Kohen is prohibited from becoming Tamei through a "Cherev k'Chalal" as if it were a corpse, he should not be allowed to enter any house which once contained a dead person because all of the utensils in the house should become like the Mes itself because of "Cherev Harei Hu k'Chalal"! Accordingly, there will be virtually no house in the world into which a Kohen may enter.
According to the Ramban, who explains that a Cherev is not Metamei through Ohel, there is no problem for a Kohen to enter any house (as long as he does not touch any utensils there). (See following Insight.)
(d) The RAMBAM (Hilchos Tum'as Mes 5:3 and 13) writes that a Cherev is Metamei only through Maga but not through Masa and Ohel. His reasoning is that the Halachah of Cherev teaches only that a utensil is like a Mes with regard to what the Torah specifically teaches about the way a Mes is Metamei, and the only explicit statement the Torah makes about Tum'as Mes is that it is Metamei through Maga (Masa is derived from a Kal va'Chomer; see Hilchos Tum'as Mes 1:2). A Cherev is not Metamei through Tum'as Ohel because the verse limits Tum'as Ohel to "Adam Ki Yamus b'Ohel," as explained earlier.
This is also the opinion of RASHI in Chulin (71b) who gives a different reason for why a Cherev is not Metamei through Masa. Rashi writes that the only Tum'ah which is Metamei through Masa is a Tum'ah which comes out of the object itself, and not a Tum'ah which the object acquires from something else.
(e) According to all of the opinions mentioned above, a Cherev is like the Mes which it touched at least with regard to being Metamei another object for seven days and necessitating Haza'ah on the third and seventh days. However, the RAMBAN (loc. cit.) and the BA'AL HA'ME'OR (cited by the Gilyon ha'Shas here) write that although the object becomes Tamei for seven days, nevertheless its Tum'ah departs without Haza'ah. This opinion answers the question posed earlier: the Gemara says that when the Nazir touches the utensils which were in contact with a Mes he certainly does not need Haza'ah, because even if they are Tamei with "Avi Avos ha'Tum'ah" and are Metamei him for seven days, they do not necessitate that he have Haza'ah.
However, the Ramban himself in Bava Basra and the TOSFOS RID here cite the Yerushalmi (Nazir 7:4) which clearly states that one who touched a Kli which touched a Mes does need Haza'ah on the third and seventh days because of "Cherev Harei Hu k'Chalal." Nevertheless, the Ramban writes that the Gemara here apparently argues with the Yerushalmi.
The RAMBAM (Hilchos Nezirus 7:8) preserves both the ruling of the Yerushalmi and the ruling of the Bavli as explained by the Ramban. The Rambam suggests that although touching a "Cherev k'Chalal" normally does require Haza'ah, if a Nazir touches a Cherev he does not need Haza'ah. The Rambam justifies this distinction logically. He writes that the Torah does not require Haza'ah for the Nazir in order for the days of his Tum'ah to count towards his days of Nezirus (because if the Tum'ah would require Haza'ah, the days of his Tum'ah would not count towards the days of his Nezirus since he would be "Mechusar Ma'aseh" in his Taharah process). (See RADVAZ on the Rambam there, and the ME'IRI here. See also Insights to 16:1 in the name of the Rambam in Perush ha'Mishnayos.)
3) HALACHAH: "TUM'AS KOHANIM" FOR "CHEREV HAREI HU K'CHALAL"
OPINIONS: The Rishonim discuss various applications of the principle, "Cherev Harei Hu k'Chalal" (see previous Insight). They discuss the degree to which this principle applies to the prohibition of Tum'as Mes for a Kohen. Is a Kohen permitted to make himself Tamei to a Cherev (a utensil or instrument which touched a Mes; according to some Rishonim, this refers only to a metal Kli), or is he prohibited from becoming Tamei to such a utensil just as he is prohibited from becoming Tamei to a Mes itself?
(a) RABEINU CHAIM KOHEN (in Tosfos DH Ta Shema) cites a Beraisa in Maseches Semachos which states that a Kohen is prohibited only from becoming Tamei to a type of Tum'ah which requires a Nazir to shave. According to most Rishonim, a Nazir does not shave for the Tum'ah of Cherev (as the Mishnah here implies, in contrast to the view of Rabeinu Tam -- see answer (a) of previous Insight). Therefore, a Kohen is not prohibited from becoming Tamei to a Cherev. This is the view of many other Rishonim as well (see TOSFOS to Shabbos 152b, and RA'AVAD, Hilchos Tum'as Mes 12:6).
The words of Rabeinu Chaim Kohen imply that a Kohen is not prohibited even mid'Rabanan from becoming Tamei to a Cherev, since he uses this logic to permit a Kohen to walk into a house which once contained a Mes, even though that Mes was Metamei all of the utensils in the house.
The Rishonim (see TOSFOS to Berachos 19b and others) point out that the Beraisa quoted earlier in Nazir contradicts the Beraisa in Maseches Semachos. The Beraisa earlier derives from the verse, "Al Kol Nafshos Mes Lo Yavo" (Vayikra 21:11), that a Kohen is prohibited from becoming Tamei to a Revi'is Log of blood. According to the Beraisa in Semachos, it must be that a Revi'is Log of blood is a type of Tum'ah which requires a Nazir to shave. However, the Mishnah earlier (49b) clearly states that a Nazir shaves only for becoming Tamei from a Chatzi Log of Dam.
The Rishonim answer that the Beraisa in Semachos means that a Kohen may become Tamei to the type of object for which a Nazir does not shave. Since a Nazir is required to shave for the Tum'ah of blood when there is a significant quantity (a Chatzi Log), a Kohen may not become Tamei to blood, even a Revi'is. In contrast, there is no type of utensil which touched a Mes which requires a Nazir to shave.
(See, however, TOSFOS RID to 54a who writes that the Tum'ah of Revi'is Dam is only mid'Rabanan. This implies that mid'Oraisa a Kohen indeed is prohibited from becoming Tamei only to a Chatzi Log of blood. When the Beraisa says that he may not become Tamei to a Revi'is Dam, it refers to the Isur d'Rabanan.)
The RA'AVAD (Hilchos Tum'as Mes 3:3) writes that a Nazir must shave when he becomes Tamei through Ohel with a Revi'is Log of blood. This view provides a simple answer to the question of Tosfos.
(b) Others rule that although a Kohen is not prohibited mid'Oraisa from becoming Tamei with a Tum'ah which does not require a Nazir to shave, nevertheless he is prohibited mid'Rabanan from becoming Tamei to such a Tum'ah (BEIS YOSEF YD 369). This might also be the intention of the RAMBAN (Bamidbar 19:14).
(c) RABEINU TAM cited by Tosfos in Berachos (19b) and Shabbos (152b) rejects the Beraisa in Semachos based on the Beraisa which says that a Kohen is prohibited from becoming Tamei to a Revi'is of blood even though a Nazir does not shave for the Tum'ah of a Revi'is of blood. Rabeinu Tam rules that a Kohen is prohibited mid'Oraisa from becoming Tamei to a Cherev and to other Tum'os for which a Nazir does not shave (such as a Golel and Dofek).
(See OR SAME'ACH, Hilchos Avel 3:2, and RABEINU CHAIM HA'LEVI, Hilchos Tum'as Mes 7:4.)
HALACHAH: The TUR (YD 369) rules that a Kohen is prohibited from becoming Tamei to a Golel and Dofek. The Tur implies that the prohibition is mid'Oraisa, like the view of Rabeinu Tam rules (in (c) above). Accordingly, a Kohen should also be prohibited mid'Oraisa from becoming Tamei to a utensil which touched a Mes.
However, the BEIS YOSEF writes that a Kohen is prohibited only mid'Rabanan from becoming Tamei with a Tum'ah for which a Nazir does not shave. In the SHULCHAN ARUCH, when he quotes the words of the Tur, he presumably means that the prohibition is mid'Rabanan (as in (b) above).
The REMA does not comment about whether or not a Kohen may become Tamei to a Golel and Dofek, but he writes that whether or not a Kohen may touch a utensil which touched a Mes is a Machlokes Rishonim. He adds that the common practice is to be lenient and to permit a Kohen to touch such a utensil. (It is unclear whether the Rema permits the Tum'ah of a Golel and Dofek for this reason as well.)

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