Mikvaos, Chapter 10, Mishnah 7. This mishnah steps away from the mechanics of immersion and records a rabbinic decree concerning a person who consumes food or drink that is tamei, together with a comparison between those laws and the laws of a mikveh.
The Chachomim decreed that one who eats ochlin temei'im, tamei foods, or drinks mashkin temei'im, tamei liquids, becomes pasul with regard to terumah: his body is rendered unfit to eat terumah. This applies only if he ingested a certain minimum amount.
The shiur of chatzi pras
The quantity spoken of here is chatzi pras. In Mishnaic usage the word pras denotes a half, and what is being halved is a loaf. A full loaf, a kikar, holds mezon shtei seudos, enough nourishment for two meals; a pras, being half a kikar, therefore comes to the food of a single meal. Chatzi pras is half of that again, the equivalent of half a meal, and that is the minimum quantity of tamei food a person must swallow in order to be caught by this rabbinic decree.
The words of the mishnah
"Kol ha'ochlin mitztarfin lifsol es hag'viyah b'shiur k'chatzi pras." All varieties of tamei food combine with one another. A person need not eat a full chatzi pras of any single food; whatever tamei foods he ate join together to reach the required measure of chatzi pras and thereby disqualify his body from eating terumah.
"Kol hamashkin mitztarfin lifsol es hag'viyah b'revi'is." The same principle applies to liquids, whose measure is a revi'is. We are familiar with the seven liquids of halachah: yayin, wine; devash, honey; shemen, oil, such as olive oil; chalav, milk; tal, dew; dam, blood; and mayim, water. All of them combine to make up the revi'is which, when it is tamei and is drunk, renders a person unfit to eat terumah.
The stringency compared with mikveh
The mishnah concludes: "zeh chomer bishsiyas mashkin temei'im mibamikveh." This represents a stringency in the laws of drinking tamei liquids as compared with the laws of a mikveh. "Asah bo she'ar mashkin k'mayim": here the other liquids are treated exactly like water, so that they are all compatible and combine with one another toward the revi'is.
By a mikveh, however, the other liquids are not treated like water. They cannot be joined to a mikveh through hashaka, so if they are tamei they remain pasul and do not become tahor. And in the other direction there is also a difference: the disqualification of she'uvim, drawn water, applies only to water and not to the other liquids.