Zavim, Chapter Five, Mishnah Four
1) If part of an unclean person rests upon a clean person, or part of a clean person upon an unclean person, or if things connected to an unclean person [rest] upon a clean person, or if things connected to a clean person upon one unclean, he becomes unclean.
2) Rabbi Shimon says: if part of an unclean person is upon a clean person, he is unclean; but if part of a clean person is upon one that is unclean, he is clean.
Section one: If part of a zav, such as his finger, rests upon a clean person, or vice versa, the clean person is defiled. “Things connected” to a person refer to his hair, teeth or fingernails. They are not flesh, but they are part of a person. The mishnah teaches that even if one of these parts rests upon another person, impurity is conveyed.
Section two: Rabbi Shimon rules that impurity is conveyed only if part of the unclean person, the zav, is on the clean person. This is because this is the typical way that impurity is conveyed by a zavby him sitting or lying or resting upon something. But the opposite is not true. If a clean person rests a part of his body on a zav, he remains clean.