Midrash su Levitico 15:12

1) (Vayikra 15:12) ("And an earthen vessel which the zav touches shall be broken, and every wooden vessel shall be rinsed in water.") "earthen vessel": This tells me only of an earthen vessel. Whence do I derive for inclusion a vessel made of alum crystals? From "And an earthen vessel. "which the zav touches": I might think even if he touches it from the back it becomes tamei. But (why is a verse needed for this?) It follows a fortiori, viz.: If a dead body, the stringent instance (of tumah), does not confer tumah upon an earthen vessel from the back, then zav, the lesser instance, how much more so does it not confer tumah upon an earthen vessel from the back! — No, this may be so with a dead body, which does not effect (tumah of) mishkav and moshav, as opposed to a zav, which does effect such tumah. And since it does, I would think that since it does so, it would confer tumah upon earthen vessels from the back. It is, therefore, written (lit.,) "which the zav touches bo" ["in it"], and, elsewhere (Vayikra 6:21) (lit.,) "which is cooked bo." Just as there, the meaning is "in its atmosphere", here, too, the meaning is "in its atmosphere" (and not if he touches it from the back).

2) Now that we have learned that tumah is conferred upon it from its atmosphere (i.e., even without touching it), what is the intent of "which he touches"? As if he "touches" all of it (i.e., If he moves it without touching it) — "heset" — (all of it must be moved).

3) "and every wooden vessel shall be rinsed in water": R. Shimon said: What does this come to teach us? If that he (the zav) confers tumah upon vessels requiring rinsing, by touching (them), is it not already written (Vayikra 6 verse 7) "and he who touches the flesh of the zav shall wash his clothes"? If one who touches the zav, thereby confers tumah upon vessels requiring rinsing, then if he himself (touches them), how much more so should he confer tumah upon them! What, then, is the intent of "and every wooden vessel shall be rinsed in water"? This refers to the food, drink, and vessels on top of the zav.