There is a famous story told of Rav Yisrael Salanter, who one year insisted that everyone in his community eat on Yom Kippur due to the outbreak of a cholera epidemic. He felt that the increased risk of infection caused by fasting warranted that everyone, including those who were perfectly healthy, must eat to keep their immune systems strong. According to the popular account of this story, which appears in Rav Baruch Epstein’s Mekor Barukh (2:11), Rav Yisrael approached the front of the synagogue, and in full view of the congregation that had assembled for the Yom Kippur prayers, he recited kiddush over a cup of wine followed by the berakha of “mezonot” over some food.
Rav Asher Weiss raised several questions regarding this account. For one thing, according to the consensus of halakhic authorities, one who needs to break his fast on Yom Kippur does not first recite kiddush. Since Yom Kippur was established as a fast day, the Sages did not require kiddush over a cup of wine on Yom Kippur as they did on other festivals, and thus even if one is required to eat on Yom Kippur for health purposes, there is no obligation to recite kiddush. (See Mishna Berura 618:29.) Secondly, even if we assume that Rav Yisrael Salanter maintained, in principle, that kiddush should be recited before breaking one’s fast on Yom Kippur, it is difficult to understand how this would work as a practical matter. The well-established halakha of “kiddush bi-mkom se’uda” requires that kiddush be recited in the framework of a meal. This means that when one recites kiddush, he must then eat a halakhic meal, defined as the consumption of a ke-zayit of bread, or at least a ke-zayit of “mezonot” food (or a revi’it of wine). This quantity must be eaten within several minutes (4-9 minutes, depending on varying opinions) of reciting kiddush. However, when an ill patient must eat on Yom Kippur, he must, if medically possible, eat in small installments. Practically speaking, then, it seems difficult to imagine Rav Yisrael Salanter eating a complete ke-zayit of food within the required time-frame after kiddush on Yom Kippur. It is hardly conceivable that a healthy person seeking to maintain his immune system to avoid infection would have to eat a sizable portion in one sitting, and could not maintain his health by spreading his eating over numerous small installments. There thus seems to be no halakhic possibility of reciting kiddush in such a case.
Indeed, Rav Weiss cites other sources that record the story differently. One particularly interesting version appears in Rav Betzalel Zev Shafran’s Shu”t Ha-Rabaz (11), where he writes that he heard from reliable sources that Rav Yisrael Salanter advised his community to take a formal vow before Yom Kippur rendering their food forbidden for consumption. Once the food was prohibited by force of the vow, the food could not become subject to the prohibition against eating on Yom Kippur, in light of the halakhic rule of “ein issur chal al issur” – an object forbidden by force of one prohibition cannot then become forbidden by force of a second prohibition. This way, the food would be forbidden on Yom Kippur by force of an ordinary Torah prohibition, and not by force of the especially severe prohibition against eating on Yom Kippur. Although Rav Yisrael maintained that eating was necessary to avoid infection, nevertheless, he sought to lower the stakes, so-to-speak, by having the food forbidden by force of an ordinary Torah prohibition. (After citing this account, Rav Shafran then proceeds to question this recommendation.)
Regardless, it seems that different accounts of this episode exist, and it is difficult to ascertain how Rav Yisrael Salanter actually ruled. Rav Weiss makes this point in response to a suggestion made by a certain to allow eating on fast days during flu epidemics, based on this well-known story of Rav Yisrael Salanter. Rav Weiss said that as the details of this story are uncertain, no definitive conclusions can be reached on this basis.