A disease is a particular abnormal condition that negatively affects the structure or function of part or all of an organism, and that is not due to any external injury. Diseases are often construed as medical conditions that are associated with specific symptoms and signs. A disease may be caused by external factors such as pathogens or by internal dysfunctions.
SHAFAQNA – Imam Sajjad (AS) had severe stomach ache in Karbala to an extent that he could not carry weapons or fight [1,2,3]. The illness started in Karbala and not from the time Imam Hussain (AS) started his journey towards Iraq. Of course his illness was a divine plan so that one child would remain from Imam Hussain (AS) to undertake the Imamate and the leadership of Muslims.
 Basaerul Darajat, Saffar Qomi, Pages 148 and 163.
 Alershad, Sheikh Mofeed, Page 114.
 E’alammul Wara, Tabarsi, Page 246.
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Thank you for your question. Illness, as well as other types of ordeal, covers a variety of purposes including the forgiveness of sins, an increas in thawab and an increase in stations.
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the ahadith in this regard please see the following podcast episodes:
May you always be successful
Imām Sajjad (alayhi salam) taught us a number of different supplications. They are all found in The “As-Sahifa al-kamilah al-Sajjadiyya”, and also available at https://www.al-islam.org/sahifa-al-kamilah-sajjadiyya-imam-zain-ul-abide....
Imām ’Ali (alayhi salam) recited the following dua when he was sick:
In the name of Allah the Beneficent the Merciful.
O Allah! Whenever You bestow a bounty upon me, my thanks is always less and whenever You have tried me in calamities, my patience was always less.
Then O the one who never made my thanksgiving being low, and O the one who, when sent the calamities my patience was very less inspite of which
He never deprived me of anything and O He who sees me doing sins but never degraded me.
And O He who saw me openly committing sins but never punished me.
Send blessings on Mohammad and his progeny, and grant me cure from this sickness without a speck of doubt You have power over all things.
You may find his supplications at the “As-Safiha Alawiya” .