Zavim, Chapter 5, Mishnah 8. The mishnayos of this chapter are engaged in a kind of reckoning: given a particular source of tumah, how many further stages of contamination can the person who contracted it pass along? The sources treated up to this point were avos hatumah whose tumah reaches a person through maga, plain physical contact, leaving him tamei to the degree that his garments are affected as well. Our mishnah moves on to a second family of avos hatumah, ones that likewise leave a person able to contaminate clothing, yet which do their work without any touching whatsoever.
The merkav of a zav
The opening words are "HaNosei es hamerkav", one who carries the merkav. A merkav is a saddle, an object made to be ridden upon. When a zav sits upon such an item he places tumas midras upon it, and it becomes a merkav haZav, a saddle bearing his tumah. All this was already treated in the second perek of our maseches. A merkav haZav ranks as an av hatumah, and it renders tamei anyone who lifts or supports it; throughout the time he is doing so, his clothing too is subject to contamination from him.
The wording here is exact. To "carry" the merkav includes supporting its weight even without shifting it from its place, since a merkav haZav transmits tumah through masa, through being borne. Simply holding it aloft is thus sufficient to render a man tamei and to leave him capable of passing tumah on to garments.
A second case follows: "hanisa alav", one who is borne upon the saddle, that is, a person who himself presses midras upon this merkav haZav. He likewise contracts tumah, and for as long as he stays there his clothing is affected by him as well.
Then a third: "v'hameisito", one who moves it. The reference is to a person who causes it to shift without laying a hand on it and without carrying it, say by prodding it along with a stick. Merely putting it into motion is sufficient for the tumah to take hold of him.
Metamei shnayim u'posel echad
Concerning all three the mishnah rules "metamei shnayim u'posel echad", they contaminate two and invalidate one. It makes no difference whether he supports the saddle, sits upon it, or shifts it: in each case he is reckoned as a man whose tumah came from an av hatumah and who can still affect garments. Clothing and keilim he handles become a rishon l'tumah; those items turn food into a sheini l'tumah; and such food, as a shlishi, disqualifies terumah. Two stages of contamination, then a stage of invalidation.
The mishnah then adds: "peirash, metamei echad u'posel echad". From the moment he has left the saddle behind, his power over clothing lapses, for the link to the source of his tumah has been broken. What remains of him is only the level of one whose tumah derives from an av hatumah, namely a rishon l'tumah. In that capacity he turns food into a sheini l'tumah, and that food goes on to disqualify whatever terumah it meets.
A neveilah
Next the mishnah says: "HaNosei es haneveilah", one who carries a carcass. A neveilah is the corpse of an animal, and its species is immaterial: kosher or not, the tumah is the same, whereas meat that underwent proper shechitah is naturally an altogether separate subject. Whoever carries such a carcass contracts tumah, and while the burden is still upon him he can convey tumah to garments, including the very clothes on his back.
Mey chatas
The last case of the mishnah is one who bears "mey chatas she'yeish bahen k'dei hazayah", purification water in a measure adequate for sprinkling. Some explanation is called for. The Torah's remedy for tumas meis, the tumah imparted by a human corpse, whether it has settled upon people or upon keilim, is hazayah: sprinkling with water into which the ashes of the Parah Adumah, the red cow, have been mixed. That liquid is termed mey chatas, and also mey niddah, water of sprinkling. In practice one would take stalks of eizov, hyssop, immerse their tips in the water, and then flick the drops clinging to them upon the person or the vessel requiring purification.
Our mishnah, then, is discussing an amount of this water sufficient to accomplish such a sprinkling. Whoever bears or shifts water of that measure becomes tamei, and for as long as it is upon him his garments too can be contaminated through him.
The same reckoning governs both of these final cases: "metamei shnayim u'posel echad", two levels of contamination and one of invalidation, along the lines set out above. And "peirash, metamei echad u'posel echad": with the connection severed, he stands as a rishon l'tumah, contaminating one level and invalidating the next, turning food into a sheini l'tumah, with that food disqualifying terumah that comes into contact with it.