We still discuss sickness that drops the obligation of fasting. You mentioned the categories of sickness including severe, slight, and mild. But who decides to which category the sickness belong to?
To determine such a point, reference must be given to experts or the sick person himself. This has become clear in this age as we have doctors. The issue is how to ascertain the presence of the effective cause of the ruling; that is, to decide whether this disease remits the obligation of fasting or not. There are diseases such as diabetes or nephritis where one may not fast. It is the doctor here who determines the level of seriousness that gives license to the person or even makes it obligatory upon him to break his fasting.
I should repeat the word obligatory in this context, because a lot of people are not convinced to give up fasting and think that they thus do an act of seeking nearness to Allah while they really do not, because Almighty Allah has remitted such a person from the obligation of fasting and thus breaking his fast has become the obligation he has to fulfill.