The Gemara in Masekhet Megilla (5b) records a tradition that Rebbi (Rabbi Yehuda Ha-nasi) wanted to abolish the observance of Tisha B’Av, but the other sages opposed his position. Initially, this was understood literally, to mean that Rebbi sought to do away altogether with the observance of this day as an occasion of mourning. But it was then clarified that in truth, Rebbi sought to suspend the Tisha B’Av observance only in the specific case when the 9th of Av falls on Shabbat. He felt that once Tisha B’Av cannot be observed on its actual date, due to the celebration of Shabbat, it should not be observed at all in such a year. The other sages, however, felt that in such a case Tisha B’Av should be observed on Sunday, instead of being suspended, and this is, of course, the accepted practice.
Tosefot raise the question of how the Gemara could have even entertained the possibility that Rebbi sought to abolish the observance of Tisha B’Av altogether. The Gemara elsewhere (Ta’anit 30b) notes the vital importance of mourning the fall of Jerusalem, and besides, Rebbi would not have had the authority to overturn the enactment issued by the sages of earlier generations to establish this observance. Tosefot’s first answer is that Rebbi sought not to abolish the entire observance of Tisha B’Av, but rather to repeal the unique measures of stringency that apply on this day. Tisha B’Av is just one of four fast days that we observe in commemoration of tragic events relating to the destruction of Jerusalem (the others being Tzom Gedalya, Asara Be’Tevet, and Shiva Assar Be’Tammuz). All four fasts are mentioned together in a single verse by the prophet Zecharya (8:19), suggesting that they are all of equal stature, yet Tisha B’Av is singled out for several special stringencies. Tosefot therefore suggest that Tisha B’Av was initially instituted as a fast no different than the other three, without any special measures of stringency, and it was later generations that began observing the stringent practices that are followed until today. Accordingly, Tosefot propose, the Gemara initially thought that Rebbi sought to abolish not the entire observance of Tisha B’Av, but only the practices that set it apart from the other three fasts.
Tosefot’s comments are significant in that they point to two different tiers of prohibitions on Tisha B’Av. The initial formal institution of Tisha B’Av, according to Tosefot, included only the requirement to fast similar to the other three fast days, beginning from daybreak in the morning. It is only by force of accepted custom that we begin the observance at sundown the previous day, and that we abstain from other forms of physical comfort and enjoyment (bathing, applying ointment, marital relations, and wearing shoes).
The precise opposite view is taken by the Ramban, in his Torat Ha-adam, as cited in Beit Yosef (O.C. 550). The Ramban claimed that to the contrary, all four of these fasts were initially established on the same level of stringency that is now observed on Tisha B’Av. This means that originally, after the fall of the First Commonwealth, it was forbidden even on the other three fasts to bathe, wear shoes, and so on, like on Tisha B’Av. Additionally, all four fasts began at sundown the previous day. However, when the Jews returned to the Land of Israel and built the Second Temple, although they continued observing these four fasts, they lowered the level of stringency except with regard to Tisha B’Av. According to the Ramban, then, no distinction exists between the various prohibitions observed on Tisha B’Av. All are observed by force of the initial institution of Tisha B’Av as a day of mourning, and thus all, presumably, apply on the same level of stringency.
Rav Asher Weiss notes that these different perspectives might yield practical implications in the case of somebody who for health reasons cannot observe all the Tisha B’Av prohibitions. For example, somebody experiencing fatigue on Tisha B’Av might be able to continue fasting if he takes a shower, which will give him more energy. If the person is confident that he can avoid breaking his fast by bathing, then presumably, according to Tosefot, he should do so. Since the prohibition against bathing applies on a lower level of stringency than the requirement to fast, in a case where one must choose one over the other he should bathe so he can continue fasting. According to the Ramban, however, it is possible that the requirement to fast does not necessarily override the prohibition against bathing, as both were instituted together when Tisha B’Av was initially established. Therefore, a person who is forced to choose one over the other would have the option to either fast or bathe. (Rav Weiss adds, however, that one could still argue that even the Ramban would view the fasting requirement as more stringent than the prohibition against bathing, in light of the fact that on Yom Kippur, the fasting requirement is treated more stringently than the other restrictions, and on the other fast days the other restrictions do not apply at all.)
It must be emphasized that this applies only in the rare case where the patient knows for certain that bathing would enable him to continue fasting. If he is uncertain whether his condition would improve after showering to the extent that he could continue fasting, then he should eat, rather than run the risk of having to violate both prohibitions.