SHAFAQNA – The Grand Ayatollah Sistani answered a question about fasting of an ill person.
Question: Can ill person fast?
The Grand Ayatollah Sistani: If fasting of an ill person is harmful (whether it causes the illness to increase, or prolongs the illness) is not correct. Of course, in all of them, the harm must be at the level that usually cannot be tolerated, and there is no difference between being certain about the harm or assume it; even if it is probable that it causes harm (of course the probability must be in a way that causes fear and this fear is logically based; like doctor’s diagnosis or experience, etc.), and the patient must do Qadha fast for that day and Kaffarah is not Wajib for him/her.
But if the illness continues till next Ramadhan, Qadha is not necessary and must pay for everyday (missed) 750 grams of bread or macaroni (pasta) or wheat or similar to them to the poor as Fidyah (donation to the poor). And also the fasting is not correct by a healthy person who has fear of outbreak/spread of an illness, let alone to be certain about it. And if fasting has no harm for an ill person, (he/she) must fast, and the fasting is correct.
Source: PERSIAN SHAFAQNA