Zavim, Chapter Five, Mishnah Three
This mishnah compares how three objects, (1) something that a zav has sat or laid upon, (2) carrion and (3) a piece of a human corpse, defile without contact.
1) Whatever carries or is carried by objects on which one sits or lies upon remain clean, except for a person.
2) Whatever carries or is carried by carrion is clean, except for one that shifts it.
a) Rabbi Eliezer says: also one that carries it.
3) Whatever carries or is carried upon a corpse remains clean, except for one that overshadows, or a person when he shifts it.
Section one: Anything that carries an object that was laid upon by a zav without touching it or is carried by it, meaning it was above and the thing the zav laid or sat upon was below, remains clean. The only exception is a person. If a person carries or is carried by something upon which a zav sat or laid, the person is impure, even if he didn’t touch it.
Section two: The mishnah compares this situation with that of carrionmeat from an animal that had not been slaughtered properly. Whatever carries or is carried upon carrion (no pun intended, I promise) remains pure (as long as he didn’t touch it). The only exception is a person who shifts the carrion. He is impure, even if he doesn’t touch it.
Rabbi Eliezer says that also one who carries carrion is impure, even if he didn’t move it.
Section three: A corpse or piece thereof also doesn’t convey impurity to someone through carrying or being carried. But it does convey impurity through overshadowing or through shifting even if the corpse has not been touched.