Explanation (Ri): This is like we say in Nidah (22a), that Zivah in which semen is mixed cancels only that day. Zivah in which no semen is mixed cancels [all clean days counted, i.e. up to] seven days.

Rava discusses when he checked himself and found semen and also Zivah on the garment;

He does not know that if the Zivah came by itself and the semen by itself, and he cancels seven days, or if semen was mixed [with the Zivah], i.e. they came together, and he cancels only one day.

The text: The text says "what is the reason? There is no greater sickness than this!"

Explanation (Ri): A convert saw Keri before conversion, and converted, and saw Zivah. He is Mitamei through Zivah. The Keri does not exempt the Zivah;

According to Rav Ada and Rav Papa, that the reason Keri exempts Zivah that comes afterwards is due to weakness, we should say so also regarding a convert. There is no greater sickness than this!

Just like we check him (a convert) with all seven ways, so we must check him to exempt him through Keri (if he saw Keri earlier in the day);

What is the difference if he saw Keri when he was a Nochri, or if he saw when he was a Yisrael?!

Explanation: Rav Ada and Rav Papa are wrong. Surely, the reason for Keri (why it exempts Zivah) is not due to weakness. Rather, it is a Gezeras ha'Kasuv;

We expound in the Sifri "v'Hayah Lifnos Erev Yirchatz ba'Mayim" teaches that Keri exempts Zivah that comes afterwards within 24 hours. Therefore, there is no difference whether or not he already received Tum'as [Zav]. We say that Keri exempts Zivah afterwards within 24 hours.

Semen of our Mishnah does not discuss Zivah, like Rav Ada bar Ahavah said. Rather, it is like we initially said, regarding Tum'as Masa, and it is like the following Tana'im;

(Beraisa): Semen of a Zav has Tum'as Masa within 24 hours.

Explanation: This is when he saw Keri within 24 hours of seeing Zivah.

Question: Why didn't we bring the Mishnah in Kelim (1:3), which says "spit and semen of a Zav are more Tamei. They are Metamei through Masa"? This is brought in Bava Kama (25a)!

Answer: "There are Raglayim l'Davar" in our Mishnah discusses only semen seen shortly after Zivah. There (the Mishnah in Kelim) needs to bring that they have Tum'as Masa all seven days.

Explanation: [The verse discusses] a Zav who had Mikreh Laylah, i.e. he saw Keri within 24 hours after seeing Zivah. His semen has Tum'as Masa.

Citation of Gemara: And it says "Lifnos Erev."

Explanation: Even after nightfall, i.e. until 24 hours, like we expound also in the Sifri, like I explained above (DH Ela), that it connotes 24 hours.

The one who says 24 hours derives from Lifnos Erev. The one who says "that day" (until nightfall) derives from Laylah.

The other opinion, how does he explain "Laylah"? The Torah wrote Laylah, for Keri normally occurs at night.

Assertion: It seems that the other opinion [who says 24 hours] expounds Lifnos Erev like we expound in the Sifri, that Keri exempts Zivah after it for 24 hours.

Note: Tosfos Yom Tov (Zavim 2:3) asks that the Mishnah explicitly says that R. Yosi holds that Keri exempts only until night! Some say that texts often confuse "R. Yosi" and "R. Yehudah". (Tosfos' text there said "R. Yehudah.") Birkas Rosh says that he expounds like in the Sifri, that Keri exempts Zivah, but not necessarily for 24 hours, like the Sifri holds.