Tevul Yom, Chapter 2, Mishnah 1. The second perek opens with the subject of liquids: what happens to fluids that come into contact with a person who is tamei, and in particular with a tevul yom, one who has already immersed in the mikvah and needs only to wait for nightfall for his taharah to be complete. Liquids are especially significant in the laws of tumah, since ordinarily liquids that a tamei person touches become tamei and can then carry that tumah further to whatever they touch. Our mishnah asks how this works when the person involved is a tevul yom.
The Mishnah states: "mashkeh tevul yom kemashkin shehu nogei'a bahen." The liquid that issues from the body of a tevul yom, his saliva or any other fluid that comes out of him, is given exactly the same halachic status as liquids that he touches from the outside. The two cases are treated as one and the same.
And what is that shared status? "Eilu va'eilu einam metamei'in." Neither the fluids that emerge from him nor the liquids that he touches acquire a status of tumah that they could then pass along to anything else. The leniency of the tevul yom, established at the beginning of the masechta, extends to the liquids associated with him: they remain incapable of transmitting tumah onward to other objects.
This is a striking result. Under normal circumstances, liquid is one of the most effective carriers of tumah there is. Yet with a tevul yom, whose own tumah is already on its way out, the liquid that leaves his body and the liquid that his hand touches are alike in that the chain stops there. Nothing further becomes tamei through them.