It goes without saying that if a person is ill on Yom Kippur, and a reliable physician determines that fasting would endanger his life, then he must – after consultation with a Rabbi – eat on Yom Kippur. Hacham Ovadia Yosef rules that if an ill patient must eat bread on Yom Kippur, then he washes Netilat Yadayim regularly, washing his entire hand up until the wrist. Although it is forbidden to wash on Yom Kippur, washing Netilat Yadayim is permissible because it is done for the Misva, and not for enjoyment. Some Poskim maintained that in such a case one should eat bread without washing Netilat Yadayim, but Halacha follows the view that one should wash Netilat Yadayim normally before eating bread on Yom Kippur. This is also the ruling of Rav Shlomo Zalman Auerbach (Israel, 1910-1995).
The Shulhan Aruch (Orah Haim 618:10) rules that if an ill patient has to eat a meal on Yom Kippur, then he adds “Ya’ale Ve’yabo,” just as we add “Ya’ale Ve’yabo” to Birkat Ha’mazon on other Yamim Tobim. On Yom Kippur, of course, we generally do not recite Birkat Ha’mazon because we do not eat, but if one needs to eat a meal on Yom Kippur, then he adds “Ya’ale Ve’yabo” to Birkat Ha’mazon. The text to insert is “Be’yom Ha’kippurim Ha’ze Ve’et Yom Selihat He’avon Ha’ze.” If Yom Kippur falls on Shabbat, then one also adds “Reseh Ve’hahalisenu,” just as we do every Shabbat.
Hacham Ovadia Yosef (Hazon Ovadia – Yamim Nora’im, p. 308) rules that if one who recites Birkat Ha’mazon on Yom Kippur forgot to add “Ya’ale Ve’yabo,” and he realized his mistake after concluding the Beracha of “Boneh Yerushalayim” but before beginning the next Beracha, then he should insert the following text: “Baruch Ata Asher Natan Le’amo Yisrael Et Yom Ha’kippurim Ha’ze Et Yom Selihat He’avon Ha’ze Baruch Melech Mohel Ve’sole’ah La’avonotenu Ve’la’avonot Amo Yisrael U’ma’abir Ashmotenu Be’chol Shana Ve’shana Melech Al Kol Ha’aretz Mekadesh Yisrael Ve’yom Ha’kippurim.” The words “Hashem Melech Ha’olam” are not recited in this Beracha.
If one did not realize his mistake until after he completed Birkat Ha’mazon, he does not repeat Birkat Ha’mazon. This applies both to somebody who forgot to add “Ya’ale Ve’yabo,” and to somebody who forgot to add “Reseh Ve’hahalisenu” when Yom Kippur falls on Shabbat.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) addresses the question of whether it is permissible to swallow the saliva in one’s mouth on Yom Kippur. He concludes that it is preferable to be stringent in this regard and to expectorate one’s saliva rather than swallow it. Hacham Ovadia, however, disagrees, and writes that there is no question whatsoever regarding swallowing saliva on Yom Kippur, and there is no need at all to be stringent in this regard. Saliva is obviously not a “beverage,” and it is produced inside the body, and there is therefore no concern whatsoever about swallowing it on Yom Kippur.
Summary: If an ill patient must eat bread on Yom Kippur, he washes Netilat Yadayim normally, and recites Birkat Ha’mazon after eating, adding “Ya’ale Ve’yabo” and, if Yom Kippur falls on Shabbat, “Reseh Ve’hahalisenu.” If he forgot to insert “Ya’ale Ve’yabo” or “Reseh,” he does not repeat Birkat Ha’mazon. It is permissible to swallow the saliva in one’s mouth on Yom Kippur.