Negaim, Chapter 13, Mishnah 8. The chapter up to this point has been occupied with the tumah of a bayis hamenuga, the leprous house. Our mishnah shifts its attention to the metzora himself, to nigei adam, and it raises a question about location: a metzora and a tahor man both find themselves under one tree, and we want to know whose presence gives that sheltered space its halachic character.
The one who settles there is the one who counts
The ruling of the mishnah begins: "hatomei omeid tachas ha'ilan," the tomei, meaning the metzora, has taken his place beneath the tree, "vehatahor oveir, tomei" - a tahor man walks by underneath and he contracts tumah. The word omeid should not be read narrowly as standing on one's feet; the intent is that he has established himself in that spot, and it makes no difference whether he does so standing or seated.
The Rav points us to the pasuk behind this. Concerning the metzora the Torah states, "michutz lamachaneh moshavo," his dwelling is to be outside the camp, and the wording teaches that the very moshav of a metzora, the place where he settles, carries tumah. Consequently, once a tomei takes up residence under a tree, the covered area beneath that tree has become his moshav, and any tahor person who walks through that same covering is rendered tomei.
The picture changes entirely when the tomei is the one on the move and never pauses to rest under the tree. Since he never settles there, the spot never becomes his moshav in the first place. Under those circumstances the tahor man, even though he is the one sitting there with a fixed place of his own, is not affected and stays tahor.
When the roles are switched
Accordingly the mishnah goes on: "hatahor omeid tachas ha'ilan," it is the tahor man who has an established place under the tree, standing or sitting, "vehatomei oveir, tahor" - the metzora only crosses through, never halting, moving along without interruption, and the tahor man is unaffected. A major principle emerges from this: a tumah overes, tumah in transit, does not transmit through an ohel.
"Im amad, tomei." Should the tomei come to a halt, however briefly, even for no more than a second, the tahor man becomes tomei, since that fleeting pause already suffices for the metzora to take a place there and turn the spot into his moshav.
A leprous stone follows the identical rule
The mishnah concludes: "vechein be'even hamenuga'as," the identical law governs a leprous stone, and the Rav lays out the case. Picture a man transporting a stone that was removed from a house declared a bayis hamenuga, and he travels along beneath a tree under which a tahor man has settled himself. As long as the bearer keeps walking without pausing, precisely parallel to the metzora who merely crosses through, we are dealing with a tumah overes, tumah on the move, and the tahor man is not rendered tomei.
"Ve'im hinicho," if he put the stone down in that place, the tahor man becomes tomei. The Rav notes further that putting it down is not actually required: the bearer's coming to a standstill is by itself decisive. Though the leprous stone remains in his hands, the moment he stops under the tree the tahor man there is rendered tomei. Metzora or leprous stone, the criterion never changes: tumah that keeps moving leaves the covered space unaffected, while tumah that comes to rest, if only for a moment, claims that space as its own.