ציטירנדיקער פּירוש אויף ויקרא 15:10

If he carried something which was ridden upon, or if he was carried on it, or he had shifted it, he defiles at two [removes], and disqualifies [terumah] at one [more remove]; But if he became separated [from the uncleanness], he defiles at one [remove] and disqualifies [terumah] at one [more remove]. A person who carries or is carried by something upon which a zav rode upon becomes a "father of impurity." This is because it is written in the Torah that he must wash his clothes (Leviticus 15:10; see 5:1). Therefore he defiles at two removes and disqualifies terumah at one more remove. Once he separates from the source of impurity, he has only first degree impurity. Note that if he touches something rode upon by a zav, he only has first degree impurity.