Zavim, Chapter 5, Mishnah 2. This perek assembles the principles of tumah and taharah that govern the zav, setting them out clearly and then illustrating them with practical cases. Our Mishnah presents a second klal in the name of Rabbi Yehoshua and then spells out exactly what it means.
The Second Klal
"V'od klal acher amru: kol hanisa al gav hazav tamei, v'chol shehazav nisa alav tahor, chutz min haro'uy l'mishkav umoshav v'ha'adam."
Anything that is carried on top of the zav becomes tamei, even where several objects separate between it and the zav himself, since the zav's weight-bearing (massa) reaches it. On the other hand, anything upon which the zav himself is carried, without his actually touching it, remains tahor.
There are two exceptions. The first is anything fit for lying or sitting upon, upon which the zav transmits tumas midras. The second is a person: a person who bears the weight of a zav becomes an av hatumah, and once he has separated from that source of tumah he retains the status of a rishon letumah.
Keitzad: The Cases Spelled Out
The Mishnah now asks "keitzad", in order to make these rules concrete and understandable.
The zav below, the tahor above
"Etzba'o shel zav tachas hanidbach, hatahor milma'alan" - the finger of a zav is underneath a course of building stones, and a person who is tahor stands above that layer of stones. Since the zav is bearing his weight, it is as though the zav is carrying him, and he therefore becomes an av hatumah.
Accordingly: "metamei shenayim u'poseil echad". He conveys two grades of tumah and disqualifies a third. Chullin that he touches becomes a rishon letumah, that in turn makes another item a sheini letumah, and that sheini renders terumah pasul as a shelishi letumah.
"Peirash, metamei echad u'poseil echad." If he separated, whether by climbing down off the stones or because the zav withdrew the finger that lay beneath them, he is now only a rishon letumah. He therefore conveys one grade, turning chullin into a sheini, and disqualifies one, since terumah that he reaches becomes a shelishi letumah.
The zav above, the tahor below
"Hatamei milma'alan v'hatahor milmatan" - now the zav is above the layer of stones, and even if only a single finger of the tahor person is beneath that row, his body is bearing the weight of the zav. Since a person is one of the exceptions to the rule that whatever carries the zav stays tahor, here too: "metamei shenayim u'poseil echad". He becomes an av hatumah, conveying rishon and sheini to ordinary chullin food, and rendering terumah pasul at the third grade.
"Peirash, metamei echad u'poseil echad." The moment he is no longer bearing the zav's weight, his level drops to rishon letumah. In that state he passes tumah down one rung, so that chullin becomes a sheini, while terumah in his hands takes on the level of shelishi letumah, a grade that matters precisely because terumah carries kedushah.
Foods, liquids and madaf above
"Ha'ochlin v'hamashkin v'hamishkav v'hamoshav v'hamadaf milma'alan, metam'in shenayim u'poslin echad." Here the items resting above the layer of stones, with the zav's finger beneath, are things such as food and drink, items that one does not lie or sit upon at all.
Although the Mishnah's language speaks of conveying two grades and disqualifying one, most of the meforshim explain that in practice such items convey only one grade and disqualify one. That is, they hold the status of a rishon letumah, which brings chullin to the level of sheini letumah, and that sheini in turn renders terumah pasul as a shelishi letumah.
"Peirshu, metam'in echad u'poslin echad." Once they have separated, the reckoning is exactly as just described: they are a rishon letumah, chullin that they reach becomes a sheini, and terumah is thereby rendered unfit as a shelishi letumah.
Mishkav and moshav below
"V'hamishkav v'hamoshav milmatan, metam'in shenayim u'poslin echad." Here the greater part of the weight of the one who is tamei rests above, and the items fit for lying or sitting upon are below. Whatever a zav lies or sits upon becomes an av hatumah. An av hatumah makes ordinary chullin food a rishon and then a sheini letumah, and a sheini letumah renders terumah, food that carries kedushah, unfit as a shelishi letumah, a status that applies to terumah alone.
"Peirshu, metam'in shenayim u'poslin echad." Notice that nothing is downgraded here once the two are apart, unlike the earlier cases. An article that served a zav for lying or sitting remains an av hatumah even after the zav has gone and it no longer touches him at all.
Foods, liquids and madaf below
The Mishnah closes: "Ha'ochlin v'hamashkin v'hamadaf milmatan, tehorin." If the zav is above the row of stones and food, drink or madaf lies beneath, they all remain tahor.
The reason returns us to the klal with which the Mishnah opened. Whatever the zav is carried upon remains tahor, with the exception of items fit for lying and sitting upon and the exception of a person. Since food and drink are not fit to be lain upon or sat upon, they do not become tamei.