How does one recover from Karet?

Question

I know Teshuva will solve any transgression. Is there anything else that is required to heal one's self spiritually in the case of relations of a Jewish man with a Jewish woman who is a niddah?

Answer 1

Answers for if it was by accident (or if did not know the law)

Shulchan aruch yd 185.4

יִתְעַנֶּה מ' יוֹם, וְאֵינָן צְרִיכִין לִהְיוֹת רְצוּפִים, רַק כָּל שָׁבוּעַ שְׁנֵי יָמִים, כְּגוֹן שֵׁנִי וַחֲמִשִּׁי, וּבְלֵיל הַתַּעֲנִית אָסוּר בְּיַיִן וּבָשָׂר. וְאִם לֹא יוּכַל לְהִתְעַנּוֹת, יִפְדֶּה כָּל יוֹם בְּמָמוֹן שֶׁיִּתֵּן לִצְדָקָה, כְּפִי עֵרֶךְ מָמוֹן שֶׁיֵּשׁ לוֹ, כִּי עָשִׁיר יִתֵּן יוֹתֵר קְצָת מֵעָנִי, וְיֵשׁ לְהַחֲמִיר בִּתְשׁוּבָתוֹ. וְכָל הַמַּרְבֶּה לָשׁוּב, זְכוּת הוּא לוֹ. (פסקי מהרא''י סִימָן ס'). ‏

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fast for 40 days, do not need to be consecutive (days), only (at least) 2 days a week, for example Monday and Thursday, and on the night of the fast(day) wine and meat are forbidden. If unable to fast, he should redeem every day (of the 40) with money that is given for tzedoko, in proportion to the money that he has, so that ritch give a little more than the poor. Should be strict with the teshuva, whoever increases in teshuva it is a merit for him.

Answer 2

There are two points. Avera--Kapara and Tum'a--Tahara. Tum'a tahara: Boel nidda is unclean but today we do not accord a great importance to this kind of uncleanness in men^[1]. In women it is associated with a prohibition of Boel nidda. 2nd Topic: chiluke Kapara, see the Gemara in Yuma. See bellow a summary of this halacha in Rambam
Of course, intercourse with a nidda women is punishable by Karet.

See Rambam, hilchot Teshuva 1, 4:

עבר על כריתות ומיתות בית דין ועשה תשובה. תשובה ויום הכפורים תולין ויסורין הבאין עליו גומרין לו הכפרה. ולעולם אין מתכפר לו כפרה גמורה עד שיבואו עליו יסורין ובאלו נאמר ופקדתי בשבט פשעם ובנגעים עונם. במה דברים אמורים בשלא חילל את השם בשעה שעבר‏
^{If a person violates [sins punishable by] karet or execution by the court and repents, Teshuvah and Yom Kippur have a tentative effect and the sufferings which come upon him complete the atonement. He will never achieve complete atonement until he endures suffering for concerning these [sins, Psalms 89:33] states: "I will punish their transgression with a rod." When does the above apply: When the desecration of God's name is not involved in the transgression.}

The source of the Rambam is in Gemara yuma 86a.:

עבר על כריתות ומיתות בית דין ועשה תשובה - תשובה ויום הכפורים תולין, ויסורין ממרקין, שנאמר (תהלים פט) ופקדתי בשבט פשעם {רש"י: למדת שיש עבירה שצריכה יסורין ומסתברא הקל לקלה והחמור לחמורה:} ובנגעים עונם.‏
If he has committed [a sin to be punished with] extirpation or death through the Beth din, and repented, then repentance and the Day of Atonement suspend [the punishment thereon], and suffering finishes the atonement , as it is said: Then will I visit their transgression with the rod {the suffering is big for a big sin and little for a little sin (Rashi)}, and their iniquity with strokes.

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[1] See Mishna Kelim 1, 3:

לְמַעְלָה מֵהֶן, בּוֹעֵל נִדָּה, שֶׁהוּא מְטַמֵּא מִשְׁכָּב תַּחְתּוֹן כָּעֶלְיוֹן‏
A man who has intercourse with a Nidda woman can make unclean men, clothes and terra cotta vessels with touching, and men by Masa. His purity is through mikve Kasher lenidda and sundown.