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5. Bathing for healing purposes:
Introduction: The bathing prohibition which applies when bathing in hot water, as well as the custom to prohibit bathing even in cold water, does not apply in a case that one is bathing for healing purposes. Thus, the only matter which needs to be dealt with here is if doing so contains a “Medical treatment” prohibition, being that in general the Sages prohibited doing medical treatments on Shabbos, as will be explained in “The Laws of Medicine” Chapter 2 Halacha 7.
Waters that are commonly bathed in [on Shabbos]: One may bathe for healing purposes in waters of Gerar and waters of Chamson and waters of Tiberius and in the pure waters of the Mediterranean Sea even though that they are salty [and thus have a greater healing affect]. [See Q&A regarding water heated from before Shabbos]
The reason for this allowance is: because it is common to bathe in them even not for healing purposes and it is thus not evident that [one is bathing in them] with intent for healing. [See Footnote regarding if this allowance applies today]
Waters that are not commonly bathed in: However [one may not bathe for healing purposes] in foul water of the Mediterranean Sea, and not in flax water being that they are repugnant, and it is [thus] uncommon to bathe in them for non-medical purposes.
To dip and leave: However, this restriction only applies if one remains in the water [for some time], however if one does not remain in them [and rather dips and leaves] then [even in the foul waters] it is permitted being that it simply appears as if he is cooling himself off [in it]. [See Footnote regarding is this allowance applies today]
Tiberius hot springs: In places that it is common to only bathe in Tiberius hot springs for healing purposes, then it is forbidden to bathe in them on Shabbos for healing even if he does not remain in the waters [a long time]. [See Q&A]
Bathing in hot water to relieve pain: One may tell [a gentile] to bring him hot water on Shabbos from one courtyard to [another] courtyard of which do not have a Eiruv between them (or through a Karmalis, see what was written in chapter 325 [Halacha 16]) in order to bathe one who is in pain.
Summary-Bathing for healing purposes:
Bathing for healing purposes is only permitted to be done in waters which are commonly bathed in for mere pleasure or to cool off. However, waters in which bathing is only done for healing purposes, is forbidden to be bathed in for healing purposes. However, one may bathe in them for non-healing purpose, such as for Mikveh. [When permitted, it may be done even according to the custom today to avoid bathing in even cold water.]
May a person who is in pain bathe in hot water on Shabbos?
May one who showers daily and avoiding so on Shabbos will cause him extreme discomfort, shower with hot water heated from before Shabbos?
Yes, as this is similar to one who is in pain.
May one today bathes in Tiberius springs for healing purposes?
No, as today one only bathes in them for healing purposes. However, if in truth it has become accustomed to bathe in it even for non-healing purposes then it is allowed.
 Supplement from Chapter 328 Halacha 48
 So is evident from the fact that hot water heated from before Shabbos is allowed to bathe in for healing, as explained in Q&A below, as well as that it was explained in Halacha 2 that in a case of discomfort one may shower and bathe in cold water and forgo the custom. Thus, certainly here in a case of pain the prohibition and custom do not apply. Vetzaruch Iyun from Ketzos Hashulchan 138 footnote 22 where he states that the allowance to bathe for healing is not in accordance to the custom that we no longer bathe today in even cold water. However, one must say that he was not referring to the aspect of not bathing due to the custom, but due to that since no one bathes due to the custom then it appears like healing which is forbidden. However, see footnote 53 where in his conclusion he rules that one may be lenient.
 Mishneh Berurah 326:138
 Seemingly although the custom is not to bathe even in cold water on Shabbos, nevertheless for healing it is allowed. Vetzaruch Iyun.
 Although today based on the custom to not bathe in even cold water one no longer bathes in these waters on Shabbos for non-healing purposes, and it is thus evident that one is doing so for healing, nevertheless today too it remains permitted as today even according to the custom one is allowed to immerse in the water to use as a Mikveh, as well as when one feels very hot and sweaty, it is thus is once again not recognizable that one is doing so for healing. [Based on Ketzos Hashulchan 138:22]
 Lit. Water that had material soaking in it
 Although today the custom is not to bathe in these waters for cleanliness, and it is thus evident that one is doing so for healing, nevertheless today too it remains permitted as today even according to the custom one is allowed to immerse in the water to use as a Mikveh, and it thus is once again not recognizable that one is doing so for healing. However, this would only work to allow men to bathe in it as opposed to women. [Ketzos Hashulchan 138 :22]
 Admur ibid; M”A 328:49
 However the M”B writes that it is only forbidden if one remains in the waters a long time. See Ketzos Hashulchan 138:22.
 Admur 307:12; Michaber 307:5
 There it is explained that many are accustomed to be lenient to ask gentiles to bring foods through a non-Eiruved courtyard as the foods are also for the need of a Mitzvah, which is Oneg Shabbos.
 Admur 328:48
 See Introduction.
 Rav Akivah Eiger 307:5 and 326:1 based on Michaber in 307:5 and Admur in 307:12; Teshuvos Radach 22; Biur Halacha 326:1 “Bemayim”; Kaf Hachaim 326:4; Ketzos Hashulchan 133 footnote 1; Shabbos Kehalacha 18:38; Lehoros Nasan 6:10; Piskeiy Teshuvos 326:1
As the Michaber in 307:5 and Admur in 307:12 rule that one may ask a gentile to carry hot water for oneself if he is pain so he can bathe in it. We thus see that it is permitted to wash in hot water to remove pain. [Rav Akivah Eiger 307:5] Now although the Michaber here does not mention hot water, it is mentioned in Admur and in the Rambam. However perhaps one can say the Michaber and Admur refer to washing minority of one’s body with this water and not majority of the body, and hence there is no proof from this halacha. [See Tehila Ledavid 326:1; Shabbos Kehalacha ibid Biurim 6] However the Teshuvos Radach ibid interprets this Halacha of the Michaber to refer to the entire body, and so is proven to be the understanding of all the Poskim above. [See Tehila Ledavid and Shabbos Kehalacha ibid]
 Rebbe Akiva Eiger in 307:5; Shabbos Kehalacha ibid; Vetzaruch Iyun as to why water heated on Shabbos is forbidden in a case of pain, and what is the source for saying this.
 The reason: As not bathing in hot water on Shabbos is a weaker Shevus than other Shevusim and is hence permitted in a case of pain. [Rav Akivah Eiger 307:5] Alternatively the reason is because one is Now, regarding why here in the above Halacha no mention is made of water heated from before Shabbos, simply the reason for this is because this Halacha is addressing the issue of healing on Shabbos, which is only problematic by springs and other bodies of water as opposed to hot water found within a vessel. Thus, regarding the bathing prohibition, in truth it does not apply at all when a person is in pain, and one may bathe in water heated before Shabbos to release pain.
 SSH”K 14:1
 Ketzos Hashulchan 138 footnote 22