What is the limit of illness that permits one to be absent from congregational prayer, knowing that the mosque is a fifteen-minute walk away, I have no vehicle to transport me, and going to the mosque causes a relapse after improvement?

If it is difficult to go to the mosque on foot due to illness, and the doctor advises rest, and attempting to walk harms him, then he is excused from attending the congregational prayer, especially if he cannot ride. The illness that excuses one is that which makes going difficult, even if it is not severe. However, if he can ride for a fare and it does not cause him undue hardship, then the congregational prayer becomes obligatory upon him. He can attain the virtue of congregational prayer in his home by praying with his family.