One who leaves his home to perform a mitzva, such as attending to a patient in the hospital, is exempt from sukka. There is a general principle that “one who is engaged in one mitzva is exempt from another mitzva” (“Osek be-mitzva patur min ha-mitzva”). The bother of seeking or walking to a sukka may impair his fulfillment of the mitzva he is already engaged in (Sukka 25a). Even if there is a sukka nearby, if there is concern that he will not sleep well there, he should sleep where he will sleep best. However, if he can fulfill the mitzva of sukka with no trouble, and it will not impair his fulfillment of the mitzva he is already engaged in – for example, there is a comfortable sukka nearby – then when the patient does not need him close by, he should eat and sleep in the sukka (SA 640:7; MB ad loc. 37-38).
Soldiers who are on guard duty and have no free time are considered to be engaged in a mitzva and need not go to the trouble of building themselves a sukka. However, their commanders, who are tasked with seeing to their wellbeing, should make sure to put up a comfortable sukka in which the soldiers can eat and (security permitting) sleep.
As a rule, one may not work on Ḥol Ha-mo’ed. However, there are exceptions to this rule, like people who work in bakeries and dairies (Peninei Halakha: Mo’adim 11:3). In these cases, if going to the sukka during work hours is troublesome for a worker, he is exempt from the mitzva of sukka due to the principle of “‘teshvu’ – ke’ein taduru.” Just as all year long they do not take the trouble to eat in a set dining room, so too on Sukkot, they are not required to take the trouble to eat in a sukka. If all year long they would prefer to eat in a cafeteria if there is one nearby, so too on Sukkot, they must eat in a sukka if there is one that they can use without troubling themselves.
Similarly, if one must take a business trip on Ḥol Ha-mo’ed to avoid the loss of a significant amount of money, and it would be troublesome for him to find a sukka while traveling, he may eat outside a sukka. If he is traveling by day, he is obligated in the mitzva of sukka at night. However, if putting up a sukka or traveling to one will take hours and harm the goals of the trip, he is exempt at night as well (SA and Rema 640:8; Levush; BHL s.v. “holkhei”).
One who must have a medical procedure on Sukkot that will cause him pain, to the point that he is considered a mitzta’er, is exempt from sukka as long as the pain endures. However, if the treatment can be performed before or after Sukkot, and he nevertheless decides to do it on Sukkot, then even though he is in pain, he is obligated in sukka. Since he unnecessarily inserted himself into a situation that makes him a mitzta’er, he is not exempt (Or Zaru’a; Hagahot Asheri; Rema 640:3).
