Our Gemara digresses from the discussion of messengers who deliver giṭṭin from the Diaspora to Israel, and raises a number of other unrelated issues. These range from a question about the height of a fence that separates between two reshuyot – public and private domains – when there is a height differential between two areas with regard to the laws of Shabbat, to several discussions about laws of tumah ve-taharah.
In order for a mikvah to be valid, it needs to have a measure of 40 se’ah of mayim hayyim – water that was collected naturally, i.e. water that was not drawn. Such water can be found in natural settings, like lakes or oceans, or collected rainwater that is directed (but not carried) into a place where it can be used for ritual immersion. Of course, regular drawn water is not, in and of itself, tameh unless it becomes tameh through contact with a dead animal, person, etc.
One surprising halakha that we find discussed on our daf is the rule that a person who enters a pool of mayim she’uvin – simple water that was drawn, and therefore cannot be used for a mikvah – or has such water poured on him, becomes tameh! The Gemara in Massekhet (14a) explains that this gezerah stems from the fact that, in the time of the Mishna, mikva’ot were often made from collected rainwater that became dirty and developed a bad smell over time. People who went to the mikvah would then bathe or shower in clean mayim she’uvin in order to cleanse themselves after tevilah (immersion). This led to people mistakenly believe that it was necessary to both dip in the mikvah and cleanse yourself afterwards to become tahor; eventually many came to believe that the mikvah was unnecessary. To counteract this belief, the sages ruled that washing after the mikvah would make a person tameh.