When the Prophets and Sages instituted these fasts, they did so for healthy people, not for the sick. This is the difference between Yom Kippur and all other fasts. On Yom Kippur, even the infirm are obligated to fast, because it is a Biblical command. Only people whose lives may be in danger if they fast are exempt, for the preservation of human life overrides the Torah’s commandments. On the other fasts, however, which were instituted by the Rabbis, anyone who is sick, even if his condition is not life-threatening, is exempt from fasting.
In general, people whose pain or weakness precludes them from continuing their regular routine of life, forcing them to lie down, are considered sick. For example, those who have the flu, angina, or a high fever need not fast.
Almost everyone develops a headache and feels weak on a fast day, and most people find it easier spending the day in bed than continuing to function normally. Sometimes, a person who is fasting even feels worse than a flu sufferer. Nonetheless, such feelings are not considered a sickness, rather the natural effects of fasting, which will pass within a few hours after the fast is over. Therefore, only one who needs to lie down because of an illness is exempt from fasting. One who suffers from the fast itself, however, must continue to fast even if his weakness causes him to prefer to lie down in bed. Only one who becomes so weak from the fast that he leaves the category of a suffering faster and enters that of the infirm may break his fast.
In addition, anyone who knows that fasting can cause him to fall ill need not fast. For example, someone who suffers from an active ulcer or severe migraines is exempt from fasting, because it is liable to precipitate his illness. Similarly, a weak person who knows that there is a good chance that he will become ill if he does not eat is exempt from fasting. Diabetes sufferers who need to take insulin need not fast, and some of them are even exempt from fasting on Yom Kippur. Those who have kidney stones are exempt from fasting, because they have to drink a lot of water. A person with high blood pressure is not considered sick and should fast, unless his doctor instructs him otherwise. Whenever in doubt, consult a God-fearing doctor.
One who is exempt from fasting because of an illness may, le-chatchila, eat a full meal and drink as much as he needs starting from the morning, but it is appropriate not to indulge in delicacies. Such a person need not eat a little at a time, as is the rule on Yom Kippur. Since that fast is Torah mandated, and even the sick are obligated to fast, the Rabbis were strict with those who are dangerously ill and need to eat, demanding that they eat less that the proscribed measure, if possible, to avoid breaking the fast. On the Rabbinic fasts, however, the infirm are not commanded to fast and they therefore need not eat a little at a time.
It is also important to note that sick people who need to take medicine regularly, like a person who has started a regimen of antibiotics or one who suffers from a chronic disease, must continue taking their medicine even on a fast day. If possible, one should swallow it without water. Realize that almost no medicine, including antibiotics, does any harm to those who take it without water. One who cannot swallow pills without water should add something bitter to the water, until it becomes undrinkable, and use it to swallow the pill.