121:9 Another leniency of the first three fasts is that pregnant or nursing women need not fast because they would suffer. Also a sick person need not fast, even if doing so would not place him in danger. In any case, one who is permitted to eat should not do so for pleasure; he should only eat what is necessary to remain healthy. Similarly, even though children do not have to fast, if they have the capacity to mourn, then it is proper to train them to eat only bread and water (and similar simple foods – Mishnah Brurah 550:5) so that they should mourn with the community.
121:10 It is prohibited on every public fast to rinse one’s mouth with water in the morning. If it is possible to spit out one’s saliva, he should do so; if this is not possible, he should swallow it. This is the case even on Yom Kippur; it is permitted because one is not doing so for his own benefit. It is likewise prohibited on a public fast to taste food, even if one spits it out. On a private fast that a person takes upon himself, one may taste food and spit it out. Similarly, one may rinse out his mouth on a private fast. (If it causes one distress not to rinse his mouth out, he may do so as long as he bends over and is very careful not to swallow any; one may even do this on Tisha b’Av if he is in great enough distress, though one may not do this on Yom Kippur – Mishnah Brurah 567:11. Similarly, if one is preparing on a fast day for a meal that is a mitzvah to be held that night, he may rely on the lenient opinion and taste the food to season it, then spit it out – MB 567:6.) [Editor’s note: The leniency of tasting food that one is preparing for a seudas mitzvah does not apply on Tisha b'Av or Yom Kippur.]