Zavim, Chapter One, Mishnah Five
1) If he saw one issue which was as copious as three, lasting as long [as it takes to go] from Gad-Yav to Shiloah, which is the time it would take to bathe and dry twice, he becomes a full zav.
2) If he saw one issue which was as copious as two, he defiles [objects] on which he lies or sits and he must immerse in running water, but he is exempt from bringing a sacrifice.
3) Rabbi Yose said: they have not spoken of “one issue as copious” unless there was sufficient to make up three.
Section one: The mishnah now mentions the possibility that a man could have one issue of zov (discharge) and it would be sufficient to count as all three issues necessary to become a full zav. The discharge would have to last long enough to bathe and dry twice, once in between each issue, had they been separate. In addition, the mishnah defines this as the time it would take to walk from a place called Gad Yavan to the Shiloah spring in Jerusalem. Unfortunately, we don’t know where “Gad Yavan” is. But assumedly it is near the Shiloah spring, the source of water for the ancient city of Jerusalem.
Section two: One issue can count as two to make a man into a zav, but not a full zav. The difference between a full zav and a zav is that a full zav must bring a sacrifice at the end of his period of cleanness.
Section three: Rabbi Yose said that there is no legal consequences to seeing one issue of zov that is as copious as two. Either the issue was as copious as three, in which case he would be a full zav, or it would count as only one. This means that Rabbi Yose would disagree with most of the mishnayot that we have seen so far in this chapter.