Zavim, Chapter 5, Mishnah 3. Our mishnah opens with the words "Mipnei she'amru" ("because they said"), and most of the commentators understand this opening as a bridge back to the teaching that came just before it. There the discussion involved a zav sitting above a row of stones, with food, drink or madaf items lying underneath him. Those items stay tahor, because what a zav carries does not automatically contract tumah; only something suited for lying or sitting upon is affected by him. Food and liquid are not fit to be lain upon, so they are untouched by his weight from above. Our mishnah now carries that principle forward and applies it in three further settings.
Above and below a mishkav
Now to the ruling itself. The mishnah declares "Kol hanosei venisa", whatever does the bearing or is itself borne, "al gabei mishkav tahor, chutz min ha'adam": in the case of a mishkav its taharah is preserved, and a human being alone is excluded. Picture vessels, or foodstuffs and liquids, lying directly beneath the zav's mishkav, with something interposed so that they never actually touch it. Their taharah remains intact. It makes no difference which one supports which: without genuine contact, no tumah travels between them.
The one exception is a human being. A person who carries a mishkav, or who is carried upon it, becomes tamei even though he never touched it and even though he did not move it.
Above and below a neveila
The very same formula is then applied to a carcass. "Kol hanosei venisa", whatever bears or is borne, "al gabei haneveila tahor", and the single exclusion is "chutz min hameisit", the one who sets it moving. Once more we are describing an item stationed over the neveila or beneath it with no contact between them, and such an item keeps its taharah. A neveila does indeed render a person tamei when he carries it, yet mere placement above or below leaves keilim and food untouched by tumah.
The exception here is the meisit, the one who causes the neveila to move. Movement changes everything. If someone else placed a person upon the carcass and the carcass was thereby set in motion, that person becomes tamei.
A second view is then recorded. Rabbi Eliezer adds two words, "af hanosei", "also the one who bears it." Taken at face value, he is including even a person who holds the neveila up without budging it in the slightest, ruling that such a person is tamei nonetheless, which makes his the more stringent position. The Gemara, though, proposes a different text of his ruling, "vehu shenosei", and that tiny alteration reverses his intent completely: he would then be the lenient voice, extending the taharah even to someone literally supporting the carcass. Some take issue with that reading, but the view accepted by most of the meforshim is that bearing a neveila without causing it to move leaves a person tahor.
Above and below a meis
The closing clause treats a corpse in identical terms: "Kol hanosei venisa", all that carries or is carried, "al gabei hameis tahor", with two matters excluded, "chutz min ha'ohel, ve'adam bizman shehu meisit". Being positioned over a meis or resting under it does not, of itself, compromise an object's taharah. But an item that spreads above the corpse like a tent, or which the corpse itself overspreads, is rendered tamei through ohel. And a human being who causes the meis to shift becomes tamei too, without having touched a thing.
The thread running through all three sections of the mishnah is the same. Position above or below, on its own, does not transmit tumah. What does transmit it is contact, movement, overshadowing, and the special vulnerability of a human being.