What is meant by severe sickness is:
1- The sickness will be made worse by fasting
2- Recovery will be delayed by fasting
3- Fasting will cause intense hardship, even if it does not make the sickness worse or delay recovery
4- The scholars also included those who fear that they may become sick because of fasting.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (4/403):
The kind of sickness in which it is permitted to break the fast is intense sickness which will be made worse by fasting or it is feared that recovery will be delayed. It was said to Ahmad: When can the sick person break his fast? He said, When he unable to fast. It was said, Such as a fever? He said, What sickness is worse than fever?
The healthy person who fears that he may become sick if he fasts is like the sick person who fears that his sickness may get worse, they are both permitted to break their fasts, because the sick person is permitted to break his fast because of the fear that his fasting may make his sickness worse and last for longer, and the fear that fasting may cause sickness is similar to that.
Al-Nawawi said in al-Majmoo’, 6/261:
The person who is unable to fast because of a sickness which he hopes will go away is not obliged to fast… This applies if he encounters obvious difficulty in fasting and is not subject to the condition that he reaches a point when he is unable to fast. Rather our companions said: The permission not to fast is subject to the condition that fasting causes him difficulties that it is hard for him to bear.”
Some of the scholars were of the view that it is permissible for every sick person to break his fast and that it is not essential that fasting cause them hardship.
This is an odd view which was rejected by the scholars.
Al-Nawawi said:
With regard to the person who is slightly sick and who does not suffer any obvious hardship, it is not permissible for him to break his fast, and there is no difference of opinion among us concerning that.
Al-Majmoo’, 6/261
Shaykh Ibn ‘Uthaymeen said:
The sick person who is not affected by fasting, such as one who has a slight cold or headache, or a slight toothache and the like, is not permitted to break his fast. Even though some of the scholars allow that because of the verse (interpretation of the meaning):
“…and whoever is ill …”
[al-Baqarah 2:185],
we say that the ruling mentioned in this verse is connected to a condition, which is when breaking the fast will relieve him of hardhsip, but if fasting does not affect him, then it is not permissible for him to break the fast, and he has to fast.
Al-Sharh al-Mumti’, 6/352.