Zavim 5:5-6

Zavim 5:5

If a ritually unclean person rests on part of an object that’s fit to lie on, or if a ritually clean person does so, he (the ritually clean person) is rendered unclean. If part of a ritually unclean person rests on something fit to lie on, or if part of a ritually clean person does so, he (the ritually clean person) remains clean. We see that impurity enters and exits the object via its lesser part. Similarly, if a loaf of trumah is placed on something that’s fit to lie on and there's paper between the loaf and that object, whether the loaf is above or below the object, it remains clean. Similarly, it remains clean in the case of a stone with tzaraas, though Rabbi Shimon rules such a case unclean.

Zavim 5:6

If someone touches a zav, a zavah, a menstruant woman, a woman who has given birth, a metzora or anything on which one of these people has sat or lain, he transmits impurity to two degrees and disqualifies trumah one degree more. If he separated (from the source of impurity), he transmits impurity to one degree and disqualifies trumah one degree more. This is true regardless of whether he touched, moved, carried or was carried by that person or object.