Parah, Chapter 10, Mishnah 2. This chapter deals with the safeguards the Sages placed around anyone occupied with the chatas water, the water made with the ashes of the parah adumah. One of those safeguards is a very light grade of rabbinic tumah known as madaf, a word taken from the phrase "kol aleh nidaf" (the sound of a driven leaf): as a rustling leaf is the faintest of sounds, so madaf is the faintest of tumos. Our mishnah presents several rulings involving it.
The text of the mishnah
"Hatahor l'chatas" (one who has attained the purity required for chatas) "shenaga b'madaf, tamei": if he came into contact with madaf, he has lost that status and is tamei.
"Lagin shel chatas" (a jug holding chatas water) "shenaga b'madaf, tamei": it too becomes tamei once madaf has touched it.
"Hatahor l'chatas shenaga b'ochlin u'v'mashkin": where a person tahor for chatas handled food or drink, the mishnah distinguishes, "b'yado tamei, b'raglo tahor": contact made by hand disqualifies him, while contact made by his foot leaves him tahor.
"Hesitan b'yado": and if, without any contact, he merely displaced those items by hand, "Rabbi Yehoshua m'tamei", Rabbi Yehoshua holds that he has become tamei, "vachachamim m'taharim", while the Sages hold that his purity stands.
The madaf of a zav
Alongside the madaf connected with midras tumah as it applies to the chatas water, that light rabbinic tumah which guards the stringent purity of these waters, there is a second madaf: the items lying above a zav.
Picture a thousand mattresses stacked one on top of another with a zav sitting or reclining on the topmost one. Every one of them, including the lowest, which he never touched, becomes a primary source of tumah that renders people and vessels tamei, by force of the midras of a zav. Now picture a thousand items piled on top of the zav, even items not designed for lying or sitting. All of them contract madaf tumah, including those the zav never touched, and this light tumah is enough to render foods and liquids tamei.
In short: everything beneath him that serves for lying or sitting, even untouched, contracts midras and conveys tumah to a person who touches it and to his garments; and everything above him contracts madaf and renders foods and liquids tamei. This is the sense in which the uppermost covering of a zav is itself called madaf.
Reconciling the opening clause
The Rambam, the Rosh and the Bartenura all address a difficulty in the mishnah's first clause, and it requires some elucidation. The phrasing gives the impression that this clause simply continues the previous mishnah's subject, the madaf that pertains to chatas. But that reading creates a clash between the two mishnayos. Here the implication is that only touching madaf disqualifies a person who is tahor for chatas, while the earlier mishnah implied that madaf with regard to chatas conveys tumah even through carrying or moving.
The Gra resolves this by reading the text here as teaching that one who is tahor for chatas and carried madaf is tamei. On his reading, the previous mishnah did not state the law of madaf for chatas at all; it merely defined what does and does not qualify as madaf for chatas. Our mishnah then follows directly on the closing portion of that mishnah and supplies the law itself: madaf renders a person who is tahor for chatas tamei even through carrying, while it renders a vessel that is tahor for chatas tamei only through contact.
According to the text as we have it, however, the clause cannot be reconciled in that straightforward way. The commentators therefore conclude that our mishnah is not speaking of the madaf of chatas at all, but of the madaf of a zav, that is, the covering above him, which likewise bears the name madaf.
Why the ruling needs to be stated
One may still object: what is the novelty in telling us that the uppermost covering of a zav, tamei only by rabbinic enactment, disqualifies a person who is tahor for chatas through contact? Even a vessel that is entirely tahor, so long as it is fit to receive midras or corpse tumah, disqualifies one who is tahor for chatas through contact. The Tannaim in the previous mishnah divided only over carrying and moving, whether an item fit for corpse tumah but not fit for midras counts as madaf for chatas; regarding plain contact, all agree that it conveys tumah.
The Kesef Mishneh answers that the earlier statement, that even a tahor vessel disqualifies one who is tahor for chatas through contact, applies specifically to contact with his hand, and not to contact with the rest of his body. Our mishnah therefore teaches something new: the uppermost covering of a zav disqualifies one who is tahor for chatas through contact of any kind, including contact with his body.
The remaining rulings
The same heightened caution governs the rest of the mishnah. A jug already filled with chatas water is rendered tamei the moment madaf comes up against it. And a person holding the purity of chatas who handles food or drink forfeits that status when the contact came through his hand, whereas contact made by his foot leaves him unaffected. Where there was no contact at all and he only shifted such items by hand, Rabbi Yehoshua considers him tamei, while the Sages leave him tahor.
One principle runs through the entire mishnah: the standard of purity demanded of a person occupied with the chatas water is so exacting that a tumah as slight as the rustle of a leaf is enough to bar him from it.