Sukkah, Chapter 2, Mishnah 4. In this mishnah we will learn four topics: the validity of a sukkah whose walls are trees, the exemption of those sent on a mitzvah errand, the exemption of the sick and those attending them, and the law of eating a casual meal outside the sukkah.
"Ha'oseh sukkaso bein ha'ilanos veha'ilanos dfanos lah - kesheirah":
One who makes his sukkah among trees, and the trees serve as its walls, as the sides of the sukkah - the sukkah is valid. The Gemara qualifies this with two conditions: that the trees be sturdy enough, or that he tied them in a way that makes them sturdy enough, so that an ordinary wind will not move them; and that he filled in every gap between them, so that no breach remains that could invalidate the walls of the sukkah.
"Sheluchei mitzvah peturin min hasukkah":
People who set out on the road on a mitzvah errand, such as to redeem captives and the like, are exempt from the mitzvah of sukkah, by the rule that one who is engaged in a mitzvah is exempt from another mitzvah. They are not obligated to fulfill a second mitzvah while they are occupied with the mitzvah before them.
This exemption applies even when they make camp at night, and likewise during the day when they stop to rest. The reason: if they were required to go out and search for a sukkah, it would detract from their ability to carry out the very mitzvah for which they set out on the road.
"Cholin umeshamsheihen peturin min hasukkah":
The sick, and likewise those who serve and care for them, are exempt from the sukkah. This does not refer specifically to a person whose condition is so severe that he is completely unable to fulfill the mitzvah, for the mitzvah of sukkah is unique in its definition: from the verse "teishvu" our Sages derived "teishvu ke'ein taduru" - that a person dwells in the sukkah in the same manner that he dwells in his home.
Therefore, a person who is not feeling well, and in such a state would not remain in a house colder than usual but would move to another house - is exempt from the sukkah under these circumstances. And those caring for him are included in the exemption as well.
"Ochlin veshosin arai chutz lasukkah":
A person may eat and drink a casual meal outside the sukkah, meaning in an informal manner, a small amount to quiet his hunger. The Gemara establishes the measure: up to the quantity of an egg's volume of food, he may eat outside the sukkah.
In summary: In this mishnah we learned that trees may serve as walls for a sukkah provided they are stable against an ordinary wind and have no gaps; that those sent on a mitzvah errand are exempt from the sukkah by the rule that one engaged in a mitzvah is exempt, even when they make camp and rest; that the sick and those attending them are exempt by virtue of the principle "teishvu ke'ein taduru"; and that a casual meal, up to an egg's volume, is permitted outside the sukkah. Nevertheless, to eat all of one's meals inside the sukkah is certainly praiseworthy.