Zavim, Chapter Five, Mishnah Six
1) He who touches a zav, or a zavah, a menstruant, or a woman after childbirth, or a metzora, or any object on which these had been sitting or lying, conveys uncleanness at two [removes] and disqualifies [terumah] at one [further remove].
2) If he separated, he still conveys uncleanness at one [remove], and disqualifies [terumah] at one [further remove].
3) This is true whether he had touched, or had shifted, or had carried, or was carried.
Section one: The person who touches one of these five people (a metzora is one with skin disease, see Negaim) or something that sat or laid upon is a “father of impurity.” Anything he touches has first degree impurity and anything it touches has second degree impurity. Terumah that touches something with second degree impurity is disqualified.
Section two: When he separates from the source of impurity his level of impurity is lowered to first degree impurity. He conveys impurity at one only remove and disqualifies terumah at two removes.
Section three: The above is true no matter how he contacted the impurity. He need not actually touch the source of impurity.