Niddah, Chapter 5, Mishnah 3. Here the mishnah describes a kohen partway through a meal of terumah who suddenly feels that tumah is about to come upon him. What is he to do with the food he has not yet swallowed? The mishnah then adds a ruling about the minimum quantity of discharge that renders a man tamei.
The case. "Hayah ochel biterumah": a kohen was in the middle of eating terumah, "vehirgish shenizdaz'u evarav," and he sensed his limbs beginning to shudder. That shuddering signals that semen is about to be emitted and that he is on the verge of becoming a baal keri. The resulting tumah raises two difficulties. One is that he may no longer go on eating terumah at all. The other, more pressing, is that whatever terumah is still in his mouth will itself contract tumah, so he will have to spit it out, and holy food will be destroyed for nothing.
What he does. The mishnah rules: "ochez ba'amah," he takes hold of his organ, in order to hold back the semen and prevent it from exiting, "uvole'a es haterumah," and he swallows the terumah that is in his mouth. Presumably he will indeed become a baal keri a moment later, but by acting this way the hope is that he will not have rendered any of the terumah tamei in the process, and none of it will have gone to waste.
How much is needed. The mishnah concludes: "umitam'in bechol shehu," a man becomes tamei with any amount whatsoever. This is not limited to keri; it applies to a flow of zivah as well. "Afilu ke'ein hachardal" - even an amount like a mustard seed, which is a very tiny quantity of fluid, "uvifachos mikein" - and even less than that. In other words, even a microscopically small amount of fluid leaving a man's body carries the full tumah: if it is a zivah flow he becomes a zav, and if it is semen he becomes a baal keri.