Gemara

Question (Rav Papa): If a Ba'al Keri (one who had a seminal emission) is sick, if nine Kavim of water are poured on him, he is Tahor (i.e. he may learn Torah). If he immerses half (his body), and they pour (nine Kavim) on half, what is the law?

This question is unresolved.

Bava Kama 82a,b: Ezra decreed that a Ba'al Keri must immerse before saying Divrei Torah.

Berachos 20b (Mishnah): A Ba'al Keri thinks in his mind the blessings before and after Shema, but he does not say them.

22a (Rav Nachman bar Yitzchak): Nowadays, people are lenient like R. Yehudah ben Beseira. (A Ba'al Keri may learn, for Divrei Torah are not Mekabel Tum'ah.)

(Ze'iri): Chachamim abolished the immersion.

(R. Yanai): I heard that some are lenient (to rely on nine Kavim) and others are stringent (to immerse) If one is stringent, his days and years will be lengthened.

(R. Yehoshua ben Levi): What need is there to immerse (from Keri)?

He means, since nine Kavim suffice (for a sick person, and most Talmidim are considered weak and sick), there is no need to immerse.

R. Zeira was sitting in a barrel of water in a bathhouse; he asked his servant 'pour nine Kavim of water over me.'

Question (R. Chiya bar Aba): Why is this needed? You are in more than nine Kavim of water!

Answer (R. Zeira): Nine Kavim are like 40 Se'ah. Just like 40 Se'ah help only in one way (immersion), also nine Kavim works only in one way (pouring).

(Rav Dimi): The Heter of nine Kavim is only for a Choleh (sick person) who became a Ba'al Keri involuntarily (b'Ones), not for a Choleh who had Bi'ah.

(Ravin): Rav Asi said, this is only for one who had Bi'ah, but one who saw Keri b'Ones need not do anything.

(Rava): The Halachah is, anyone who had Bi'ah must immerse in 40 Se'ah. Nine Kavim suffice for a healthy person b'Ones. A Choleh b'Ones is totally exempt.

(Beraisa): If nine Kavim of water were poured on a Ba'al Keri, he may learn Torah. He may not teach others (or bless for them) until immersing in 40 Se'ah.

R. Yehudah requires 40 Se'ah in any case.

Amora'im argue about whether the 40 Se'ah must be in the ground, or if they can even be in a Kli. The former opinion explains that "in any case" includes Mayim She'uvim (water that was drawn in a Kli).

Rav Chama would immerse on Erev Pesach to say Berachos on behalf of others

The Halachah does not follow him (nine Kavim suffice).

Rishonim

Rif and Rosh (Berachos 13b and 3:21): Some say that they abolished the Tevilah both for prayer and for Divrei Torah. Some say that this was only for Torah, but one must immerse for prayer, or wash with nine Kavim. Rav Hai Gaon says that since the Gemara omits this (regarding prayer), we follow the custom of Yisrael: a Ba'al Keri does not pray until washing, even if he lacks water.

R. Yonah (13b): Some say that we permitted to a Ba'al Keri Kri'as Shema and all Divrei Torah. This is wrong. "He disgraced the word of Hash-m" refers to one who says Torah in filthy places. It is more severe than other Kerisos: even after Kares, the sin is still in him!

Rebuttal (Beis Yosef OC 82 DH v'a'Ha): This implies that we permit Kri'as Shema in filthy places. Berachos 24b explicitly forbids! Also, R. Yehudah ben Beseira taught that (Stam, i.e. all) Divrei Torah do not receive Tum'ah. This is when the person and area are clean, but the person is Tamei.

Hagahos Ashri: We hold like R. Yehudah ben Beseirah totally. It is a Berachah l'Vatalah to bless on immersing on Erev Yom Kipur.

Rambam (Hilchos Kri'as Shema 4:8): Most of Yisrael were unable to fulfill Ezra's enactment, therefore they annulled it. All of Yisrael learn Torah and read Shema even when they are Ba'al Keri, for Divrei Torah do not receive Tum'ah.

Rambam (Hilchos Tefilah 4:4): Any Tamei person washes his hands, like a Tahor person, and prays. Even though he could immerse and become Tahor, this is not essential. Ezra only forbade saying Divrei Torah before immersing. A later Beis Din forbade prayer before immersing. This is not due to Tum'ah and Taharah, rather, in order that Chachamim not be too frequent with their wives, like roosters.

Rambam (6): The custom spread through Bavel and Sefard that a Ba'al Keri does not pray until immersing his entire body in water, due to "prepare to greet your G-d, Yisrael." This is if he is healthy. If a Choleh saw Keri b'Ones, he is exempt.

Poskim

Shulchan Aruch (OC 88:1): Any Tamei person learns Torah, says Shema and prays, except for a Ba'al Keri. Ezra forbade him to do these before immersing, lest Chachamim be with their wives like roosters. Later, they abolished the enactment, and returned to letter of the law; a Ba'al Keri may learn, say Shema or pray before immersing or washing in nine Kavim. This custom spread.

Mishnah Berurah (1): The decree was also because Keri comes from frivolity, and Torah must be with fear and trembling.

Kaf ha'Chayim (2): The Shulchan Aruch wrote only lest Chachamim be too frequent with their wives, to teach that even today they should avoid this.

Beis Yosef (DH u'Mah she'Chosav v'Lav): The Rashba says that R. Yehudah ben Beseira totally exempts, even from nine Kavim.

Beis Yosef (DH u'Mah she'Chosav Rabeinu v'Nir'eh): The Ra'avad says that each place follows its custom. Ze'iri taught that they abolished the immersion, but we require nine Kavim.

Beis Yosef (DH veha'Rambam, citing the Rashba): Since the enactment was Batel, why does Rava, who is Basra, exempt only a Choleh b'Ones? He teaches the law for those who follow the old enactment.

Beis Yosef (DH v'Chosav Rabeinu): In Siman 241, the Tur says that it is an excessive stringency to immerse. Even the Rif, who is stringent for prayer, requires only nine Kavim. One who is stringent will be blessed.

Mishnah Berurah (2): One may not be stringent if this will cause missing the time for Shema or prayer, or perhaps even praying with the Tzibur.

Beis Yosef (DH v'Havi): Amora'im argue about whether or not the 40 Sa'im may be in a Kli, but all permit She'uvim. It was not resolved whether half-half works. This is mid'Rabanan, so we may be lenient, i.e. for one who accepted to observe Ezra's decree.

Beis Yosef (DH v'Tish'ah): The nine Kavim must be from at most three Kelim. They must be poured without break; they start pouring the new before the old ceases. They must be poured on him; it does not help to immerse in them.

Mishnah Berurah (1): There are pious people who observe the enactment. If it is difficult to immerse, they wash in nine Kavim. One stands at the time with his hands loose near his heart, not clenched. The water is poured on his body. The same applies to those who use nine Kavim to be Metaher on Erev Rosh Hashanah and Erev Yom Kipur. A Choleh who saw b'Ones needs only to wash the area. Nothing is required for clear water that came from the Ever. Nine Kavim is 24 of our quarts. One may use 18 quarts if he lacks big Kelim.

Note: According to the Chazon Ish, it is 21.6 liters. According to R. Chaim Na'eh, it is about 12.5 liters.

Kaf ha'Chayim (5): The Chida cites a tradition that if one had an emission and immersed the same day and confessed properly, the Mazik (damaging force) of Keri is wiped out. Therefore, if he cannot immerse in a Mikveh, he should immerse in 40 Sa'im of Mayim She'uvim, or pour nine Kavim on himself.