Sometimes a person becomes so weak on Yom Kippur that he is afraid that he is going to lose consciousness and die. This fear is usually exaggerated, as fasting and the attendant weakness are generally not dangerous. (There are even some serious illnesses for which fasting may be helpful.) Nevertheless, it is possible that a person does have some issue which might make fasting dangerous. Therefore, if someone is so afraid he might die that he asks for food and drink despite the holiness of the day, we give it to him. However, since the need is sometimes simply psychological, we begin by giving him only a little. Sometimes this is enough to calm him and bring about a recovery. If this does not work, we continue giving him small quantities of food, spread out over time (as explained in the previous section). If this, too, does not work, he may eat and drink until he is reassured (SA 617:2-3).
Sometimes just knowing that it is permissible to eat and drink restores a person’s well-being. He calms down and feels able to continue fasting. There is a story in the Yerushalmi (y. Yoma 6:4) about R. Ḥaggi, who became very weak from fasting, but when R. Mana told him to drink, he decided that he could manage, and he continued to fast. Many poskim use this strategy (Kol Bo §69, cited by Beit Yosef 618:1).
On the other hand, we must be very careful not to take danger lightly. If doctors have instructed someone to eat and drink, he should do so joyfully, as he is fulfilling God’s commandment to take care of his health. Hopefully, he will merit long life as a reward for observing this mitzva. Torah giants made it a practice to admonish the sick about this. If they knew that a particular patient was likely to disregard medical advice and fast, thus endangering his life, they would visit him on Yom Kippur to persuade him to eat and drink.