Zavim, Chapter Five, Mishnah Ten
1) He who touches a dead sheretz, or semen, or he that has suffered corpse uncleanness, or a metzora during his days of counting, or hatat waters of insufficient quantity with which to perform the sprinkling, or carrion, or an object ridden upon [by a zav], defiles at one [remove] and disqualifies [terumah] at one [more remove].
2) This is the general principle: anyone who touches anything that according to the Torah is a “father of uncleanness” defiles at one [remove] and disqualifies [terumah] at one [more remove], except [for the corpse] of a human.
3) If he had become separated, he defiles at one [remove] and renders [terumah] unfit at one [more remove].
Section one: All of these substances are “fathers of impurity.” One who touches one of them contracts first degree impurity. This same list was mostly found in Kelim 1:1, so one can find a more detailed explanation there. They also defile only by contact and not by carrying. Finally, none of these says that the person must wash his clothes.
Section two: The only exception to this general rule is one who touches a dead body. Such a person is a “father of impurity” and therefore defiles at two removes and disqualifies terumah at one additional remove.
Section three: In all of these cases, once he separates from the source of impurity he has “first degree impurity.” This is true even if the source of impurity was a dead body. Since he has separated from it, his impurity is reduced.