Negaim, Chapter 13, Mishnah 5. Until now the perek has been occupied with the laws of the house that has been declared a bayis hamenuga. Our mishnah turns to a different question: how does such a house pass tumah on to a person? There are two houses under discussion, the bayis hamusgar (a house placed in quarantine by the kohen, still under observation) and the bayis hamuchlat (a house whose nega has been definitively confirmed), and the mishnah tells us that they are not identical in this respect.
The mishnah reads: "Bayis hamusgar metamei mitocho, vehamuchlat mitocho ume'achorav, v'zeh v'zeh metamin b'viah." A quarantined house conveys tumah from its inside; a confirmed house conveys tumah from its inside and from its outside; and both of them convey tumah through entry.
The two kinds of contact
When the mishnah speaks of the "outside" of the house, it means touching the outer face of the wall, since with a wall there is no other mechanism available: it is plain tumas maga, tumah transmitted by touch. When it speaks of "b'viah," entry, it means that a person walked into the house with his head and the greater part of his body, rosho v'rubo, and by that act alone became tamei, without touching anything at all.
The approach of the Rav and the Rambam
The Rav (Bartenura) reads the opening clause as referring to entry. He cites the posuk, "veha'ba el habayis kol yemei hisgir oso yitma ad ha'arev," one who comes into the house all the days that it is shut up shall be tamei until evening. According to this reading, when the mishnah says that a musgar house is metamei mitocho, it is describing a person who walked inside. The Rambam explains the mishnah in the same direction.
The practical outcome of this reading is sharp: with a bayis hamusgar, only tumas biah applies. A person who enters with the majority of his body becomes tamei; a person who merely reaches his hand through the doorway and touches the inner surface of a wall does not become tamei at all.
This reading does leave a difficulty. If the opening clause already speaks of entering, why does the mishnah conclude by telling us again, "v'zeh v'zeh metamin b'viah"? That is precisely the point we were told at the outset.
The stones that carry the nega itself
The Rav adds a further line, which appears in the Rash as well, and the Tosfos Yom Tov notes that it originates in a Tosefta. There is one exception within the bayis hamusgar: the stones on which the nega itself appeared. Even while the house is only under quarantine, those stones are metamei me'achoreihem, from behind. If a person knows which stone bore the nega and touches it from the outer side of the wall, he becomes tamei.
So tumas maga does exist in a quarantined house, but only with respect to the afflicted stone itself, not with respect to the other, uncontaminated walls, whether he touches them from within or from without. And if touching that stone from the outside renders him tamei, then certainly reaching in and touching it from the inside does so as well, since that is no less a contact than touching it from behind.
The approach of Rashi
A different explanation is recorded by the Tosfos Yom Tov, and the Tiferes Yisroel adopts it as well. Its source is a comment of Rashi in Yevamos. Before presenting it he points out that Tosfos there leaves the matter somewhat unsettled, and that the wording in which Tosfos cites Rashi does not correspond to the Rashi found in our printed editions. Rashi's own language, however, states the position without ambiguity.
Commenting on the first clause of our mishnah, Rashi explains that a quarantined house transmits tumah from within by way of touch, and this holds true even where the man himself never came inside, that is, his head and the larger part of him remained outside the whole time. He extended his arm through the opening and made contact with the wall, and for that alone he should be rendered tamei under the heading of one who "comes into the house": once his hand touched the wall, the house conveys tumah to him.
This is a new dimension in the concept of ba el habayis: the combination of part of a limb entering the house together with touching the house is itself enough to render him tamei.
On this reading the conclusion of the mishnah falls beautifully into place. The reisha is teaching about touch: a musgar house transmits tumah when he touches it from within, and a muchlat house transmits tumah whether he touches it from within or from without. Only afterward does the mishnah add the separate law that both of them are metamei b'viah as well, when a person actually enters.
Why the muchlat house is stricter
About the house that has been confirmed, the Rav writes that a person who makes contact with the wall from behind is tamei. The source is the Torah's language regarding the muchlat, "tzora'as mameres... tamei hu." Why does the posuk append those closing words? To widen the scope of his tumah: contact defiles him whether his hand reached the wall within the house or met its outer surface. And of course a person who steps inside such a house is tamei too, for in the Rav's understanding even a house that is merely under quarantine conveys tumah to one who walks in.
The difference in halachah
We are left with two approaches to the mishnah, and the difference between them is a practical one, and it concerns the bayis hamusgar. According to Rashi, merely touching an ordinary wall from within, even in a house that is only under quarantine, is enough to render the person tamei. According to the Rav, and this is the Rambam's view as well, that is not so: he must actually enter the house with his head and most of his body in order to become tamei.
Both views agree, however, about the afflicted stone. Even in a bayis hamusgar, the stone that bears the nega renders a person tamei when he touches it from the outside, and all the more so when he reaches in and touches it from within.