If a person is temporarily ill, he has to make up the fast when he is able. There is no fidiya on him.

https://www.youtube.com/watch?v=hlpuHYlNRco

(1) If someone is unable to fast due to medical reason he will always give fidya (2) If someone is unable to fast due to some temporary illness or travelling he can do Qaza of the fasts or give fidya (then Qaza is not required)