Answer
Fasting is not obligatory on a person who cannot fast because of old age, or for whom fasting causes extreme hardship. He or she should give one mudd (750 grams) of food stuffs to a poor Shia Muslim for every fast.
Fasting is not obligatory on a person who suffers from a disease which causes excessive thirst, making it unbearable, or full of hardship. But in the latter case, that is, of hardship, he should give one mudd of food to poor, for every fast.