Zavim, Chapter 5, Mishnah 6. Our Mishnah steps back from the details of lying and sitting and asks a broader question: when a person comes into contact with one of the major sources of tumah, how far down the chain does his own tumah reach?
Here is how the Mishnah puts it: "Hanogei'a bazav uvazavah, uvanidah, uvayoledes, uvametzora", one who comes in contact with any of these five, the zav and the zavah, the nidah, the yoledes (a woman in the days following childbirth), and the metzora; and the Mishnah adds a sixth category of contact, "bemishkav uvamoshav", one who touches the bedding any of them lay upon or the seat any of them sat upon. In all of these cases the ruling is "metamei shenayim uposeil echad": he transmits tumah through two further levels and causes a third to be rendered unfit.
Let us unpack that. So long as this person is in contact with the source, he is treated as though he were joined to it, part of it, and he himself has the standing of an av hatumah. That is why his effect extends so far: not only he becomes tamei, but the garments he is wearing and the vessels he is holding are drawn in as well. As an av hatumah he makes what he touches a rishon letumah; that rishon in turn makes food a sheni letumah; and terumah at the next stage becomes unfit as a shelishi letumah. Those are the "two" that he renders tamei and the "one" that he renders unfit.
The Mishnah continues: "Peireish, metamei echad uposeil echad." Once he has separated himself and is no longer in contact with the zav or the nidah, his reach is shortened. He is now a rishon letumah, and so he makes ordinary food a sheni letumah, and terumah is disqualified as a shelishi letumah. One level of tumah imparted, one level of unfitness caused. These are concepts we have already encountered in our masechta, and here they are applied to the moment of contact and the moment after it.
The closing clause of the Mishnah widens the range of contact that produces this result: "Echad hanogei'a ve'echad hamaseit", whether he touched the source of tumah or jostled it, "ve'echad hanosei ve'echad hanisa", whether he bore it or it bore him. Every one of these four relationships stands on equal footing. It is irrelevant whether the item in question actually budged, and irrelevant whether any weight came to rest upon it; in each case the person walks away with the ability to convey tumah exactly as the Mishnah has taught, so that ordinary food he touches becomes tamei at its own level and terumah is left unfit for use. The Mishnah collects all of these types of contact into a single ruling, and each of them yields precisely the same chain of tumah.