Acceptable Reasons for not joining Jihad

Allay Says,

لَيْسَ عَلَى الْأَعْمَى حَرَجٌ وَلَا عَلَى الْأَعْرَجِ حَرَجٌ وَلَا عَلَى الْمَرِيضِ حَرَجٌ ...

No blame or sin is there upon the blind, nor is there sin upon the lame, nor is there sin upon the sick.

Allah then mentions the legal reasons that allow one to be excused from joining the Jihad, such as blindness and being lame, and various illnesses that strike one and are remedied in few days. When one is ill, he is allowed to remain behind and will have a valid excuse to do so, until his illness ends.

Allah the Exalted and Most honored then said, while ordaining joining the Jihad and obeying Allah and His Messenger,

... وَمَن يُطِعِ اللَّهَ وَرَسُولَهُ يُدْخِلْهُ جَنَّاتٍ تَجْرِي مِن تَحْتِهَا الْأَنْهَارُ وَمَن يَتَوَلَّ ...

And whosoever obeys Allah and His Messenger, He will admit him to Gardens beneath which rivers flow; and whosoever turns back, (from joining the Jihad and busies himself in his livelihood),

... يُعَذِّبْهُ عَذَابًا أَلِيمًا ﴿١٧﴾

He will punish him with a painful torment.

i.e. in this life with humiliation and in the Hereafter with the Fire. Allah the Exalted has the best knowledge.