Ohalos, Chapter 1, Mishnah 5. The mishnayos at the head of this perek establish how tumah passes from one item to the next and how many links that chain can hold. Our mishnah belongs to that same discussion. It sets side by side two things that can pick up tumah from a zav, a human being and a garment, and shows that each one carries a severity that the other lacks.
The tanna opens: "Adam uv'gadim mitamim bezav" - people as well as garments are subject to tumah when a zav is involved. Who is a zav? A man who has had a certain type of emission. Should he see it on two occasions, whether the two came within a single day or on two days in a row, he holds the rank of av hatumah, and every av hatumah passes tumah along to human beings and to vessels alike.
Now the comparison: "Chomer ba'adam mibab'gadim, uvab'gadim miba'adam" - in one respect the person is treated more severely than the garment, and in another respect the garment is treated more severely than the person. The mishnah proceeds to detail both.
Where the person is more severe
"Sheha'adam hanogeia bezav metamei b'gadim" - so long as a man has his hand on a zav, whatever garment he is in contact with turns tamei. Read this precisely: the halachah is not restricted to the clothes on his back. Any garment he happens to be touching at that moment is included. Picture Reuven with one hand resting on a zav and the other hand holding a garment; for as long as he keeps hold of the zav, that garment is tamei.
That should give us pause. Someone in contact with a zav has the level of a rishon, since he received his tumah from an av hatumah. And we have seen again and again throughout Maseches Keilim, and also in the earlier mishnayos here in Ohalos, that a rishon has no power over human beings or over vessels; its reach extends to ochlin umashkin and no further. How can such a man pass tumah into a garment?
The resolution: this is a din of its own, taught by a posuk. The Torah writes, "V'hanogeia bivsar hazav y'chabeis b'godov" - one who has touched the flesh of the zav is required to launder his clothing. We derive from here that for the duration of the contact, this man is able to transmit tumah to garments. Again, "garments" is not confined to what he is wearing; it covers whatever garment his hand is on at that same time.
In truth the scope is wider than the word b'gadim would indicate. Other categories of keilim are affected too: a wooden utensil in his grasp, a metal utensil under his fingers while his other hand lies on the zav. Two exceptions hold even mid-contact, namely another human being and a kli cheres. Why then did the tanna choose garments as his illustration? The Eliyahu Rabbah answers that the mishnah picked the everyday situation: clothing is what a person wears and handles all day long, so it makes the most natural example.
Set against this is the reverse case: "v'ein b'gadim hanog'in bezav", garments in contact with a zav, "mitamin b'gadim", do not pass tumah on to further garments. Lay a garment against a zav, and let a second garment rest against the first; the second one remains tahor. Nothing unusual is happening here, only the standard progression: the zav holds the rank of av hatumah, the garment touching him drops to the rank of rishon, and a rishon has no effect on keilim. Any other utensil that touched the zav follows the identical rule. Only an av hatumah has the strength to defile human beings and vessels. What sets a person apart during his contact with the zav? The verse focused on a person and on no one else, "v'hanogeia bivsar hazav y'chabeis b'godov." Nothing comparable was written about a utensil that touched the zav, so a utensil is governed by the usual sequence: av hatumah, then rishon, and a rishon can go no further than ochlin umashkin.
Where the garment is more severe
The other side of the comparison opens: "shehab'gadim hanos'in es hazav" - garments that bear the zav. Taken at face value the phrase is odd, but what it describes is weight. A garment that the zav sits down on, lies on, leans against, or presses his body upon in any fashion picks up tumas midras. When a zav loads his weight onto an object whose purpose is to be sat on, lain on or leaned on, that object is elevated to the status of av hatumah in its own right. This is midras hazav, as the posuk teaches: "V'chol hanogeia b'chol asher yihyeh tachtav", anyone touching whatever had been beneath him, "yitma ad ha'arev", is tamei until nightfall.
This language also calls for a second glance. A bench, a mattress, a cushion, a pillow: all of these plainly exist to be sat or lain upon. But a garment? Nobody manufactures clothing in order to sit on it. The mefarshim resolve this by pointing out that in the act of dressing, a person does press and bear down upon the garment, and that is enough to bring it into the category of midras.
So here is the full picture. Once a zav brings his weight to bear on an object made to hold a person, the object becomes midras, which means it is now an av hatumah. From that point on, any person or vessel that comes into contact with it descends to the level of rishon, and this remains true long after the zav has departed. Furthermore, when our mishnah speaks of a zav sitting on b'gadim, it is not restricting the din to clothing. Clothing is included, but the real category is objects intended to support a person's weight. The severity that the mishnah credits to b'gadim is in essence the severity of anything that is sat upon.
Hence the mishnah's conclusion: such b'gadim "mitamin adam" - they do transmit tumah to a human being, for that is exactly the function of an av hatumah, which reaches both human beings and vessels.
The closing words draw the contrast: "v'ein ha'adam hanosei es hazav metamei adam" - a man who bore the weight of the zav does not transmit tumah onward to another man. Should a zav lean upon Reuven, Reuven descends to the level of rishon, but he is not promoted to av hatumah. Flesh and blood never takes on midras. Consequently Reuven will not defile a second person.
One might object: did the mishnah not teach a moment ago that a man touching a zav does convey tumah to garments and utensils? It did, and the difference lies in the moment. That unique din applies exclusively while the contact with the zav is still in force. Once the zav has leaned on Reuven and moved on, Reuven is nothing more than an ordinary rishon, and a rishon touches neither people nor vessels.
Summary
Both people and garments take on tumah from a zav, and each side has a severity of its own. On the human side: throughout the time he remains in contact with the zav, though his own rank is only that of rishon, he still defiles garments and other keilim, a din we learn from a posuk. On the garment side: once the zav has pressed his weight down upon them, they are raised to the rank of av hatumah through tumas midras, and an av hatumah defiles people and vessels both. That midras status stays with them even after the zav has gotten up and gone. A person, however, whom a zav leaned upon and then left behind, is no more than a rishon, and he will defile neither another person nor a utensil.