פירוש על זבים 5:4

מקצה טמא על הטהור – even the finger of a Zav/person with flux/gonorrhea is on top of the pure person, or the finer of a pure person is on top of the Zav, or a connection, it is impure, as for example, his hair and his fingernail or his teeth, all these are called חיבורי/a connection, junction, even though there isn’t other than a minority of the impure on top of the pure person or a minority of the pure on top of the impure thing, the pure thing is defiled, for we don’t require that he carries the majority of a person with a flux/gonorrhea (i.e., the Zav) other than upon the lying but not upon a person.

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. 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.

ומקצת טהור על הטמא טהור – for we require that he carries its majority. But the Halakha is not according to Rabbi Shimon.

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. 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 zav by 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.