Allah, the Wise, says:
وَ اِذاَ مَرِضْتُ فَهُو يشْفِينِ
"And when I am sick, then He restores me to health" 
Imam A'li (peace be upon him) has said:
اَشَدُّ مِنَ الْفاَقَةِ مَرَضُ الْبَدَن
"More calamitous than poverty is the sickness of the body" 
One of the treasures of Paradise, which reaches a Mu'min in this world, is sickness. If a Mu'min, occasionally and unintentionally, falters and commits sins, God does not like him to return to Him carrying the burden of his sins; He thus afflicts him with illness in order that his sins are forgiven.
A person who suffers from illness, beseeches and supplicates to God to restore his good health and God likes this state of a sick person, for He desires His servant to converse and communicate with Him. At times, God afflicts a person with sickness in order to elevate him in spiritual rank and status.
The best amongst those who are ill is one who exercises patience in this suffering, conceals his pain and desists from complaining about his disease to others until he regains his health and acquires the maximum reward ordained for him.
1 – The Rank of a Worshipper who Suffers from Illness
One day, the Holy Prophet (s.a.w.) raised his head towards the heavens and then laughed. One of his companions asked him the reason for his laughter to which the Prophet (s.a.w.) replied saying:
"My laughter was out of my astonishment. Two angels had descended from the heavens to record the deeds of a righteous Mu'min. They always found him on his prayer mat, engaged in worship, but this time, they noticed that he was not there. He was in his bed, having been afflicted with sickness. They ascended towards the heavens and said to God: O’ Lord! We did not find Your servant in his usual place of worship but instead, found him lying on his bed in a state of sickness.
God said to them: Until he regains his health, record for him all acts of worship and deeds of goodness that he used to perform when healthy. It is necessary for Us, for as long as he remains in the captivity (of sickness), to grant him rewards of all the good deeds that he used to perform, when possessing soundness of health".
2 – My Daughter has Never Fallen Ill!
Once, the Holy Prophet (s.a.w.) sought a lady’s hand in marriage. Her father began praising her and while enumerating her virtues, he uttered:
"From the time she was born till today, she has never fallen sick."
As soon as the Holy Prophet (s.a.w.) heard this, he immediately left the gathering.
Later, he said, "There is no goodness in an entity which, like a zebra, never falls sick. Diseases and misfortunes are God’s gifts to His servants that should they ever become negligent of Him, diseases and misfortunes serve to make people mindful of Him." 
3 - Patience in Sickness
Abu Muhammad Riqqi says:
Once, I arrived in the presence of Imam Ridha (a.s.) and greeted him. He replied to my greetings, inquired after my health and began to converse with me.
In the course of the conversation, all of a sudden he said, "O’ Abu Muhammad! Every Mu'min, whom God afflicts with a misfortune and who exhibits patience over it, shall surely come to possess the rank and recompense of a martyr in the eyes of God."
I wondered, "In what connection did the Imam (a.s.) speak this? We had not been talking about misfortunes and calamities for the Imam (a.s.) to have suddenly come up with this kind of speech?"
I bid farewell to the Imam (a.s.) and proceeded towards my friends and fellow travellers, when suddenly I experienced a pain in my leg. I passed the night in great pain and when morning dawned, I noticed that my legs had become inflamed. After a period, this inflammation became more severe. I remembered the speech of Imam Ridha (a.s.) in which he had recommended patience in the face of misfortune and how I had thought it to be out of place then.
In this state I reached Madinah, but there, a large wound developed in my leg, oozing pus. The pain was so excruciating that I could not be at peace. I then realized that the Imam (a.s.) had visualized such a phenomenon when he had spoken to me and advised me to keep calm by means of patience. For ten months, I was bed-ridden as a result of this sickness.
The narrator says: After a period, Abu Muhammad regained his health only to fall ill once again, eventually passing away in that sickness.
4 – Imam Sajjad (a.s.) helps the Lepers
Once, Imam Sajjad (a.s.) encountered some lepers sitting on the roadside and eating their food. Greeting them, he passed by, when all of a sudden he stopped and said to himself:
"God does not like the haughty ones."
Having said this, he retraced his steps and approaching the lepers, said, "Presently, I am in a state of fasting (and thus unable to sit down and share your food). (However) I invite you to come to my house and be my guests."
They accepted the offer and went to his house, where the Imam (a.s.) fed them and helped them by giving them some money.
5 – The Sick Person’s Debt Paid
Usaamah Ibn Zaid was one of the companions of the Holy Prophet (s.a.w.). Once, he fell ill and so Imam Husain (a.s.) paid him a visit. Approaching him, the Imam observed him to be greatly disturbed, repeatedly moaning and exhibiting great anguish.
The Imam (a.s.) said to him, "Brother, what causes you to be so disturbed and worried?"
"I am burdened by a debt of sixty thousand dinars." he replied.
The Imam (a.s.) consoled him by saying, " I take the responsibility of clearing your debt."
Usaamah persisted, "I fear I may die before my debt is repaid."
The Imam (a.s.) said, "Do not worry! I shall clear your debt before your death." Having said this, Imam Husain (a.s.) ordered his debt to be paid off immediately.
 Holy Qur'an, ch. Al-Shua’raa (26), vs. 80.
 Nahjul Balaghah (of Faidh al-Islam), pg. 1270.
 Daastaan-ha Wa Pand-ha, vol. 6, pg. 130; Tafsir Nur al-Thaqalain, vol. 5, pg. 68.
 Pand-e-Taareekh, vol. 2, pg. 180.
 Hikaayat-ha-e-Shanidani, vol. 1, pg. 166; Bihaar al-Anwaar, vol. 49, pg. 51.
 Baa Mardum In Guneh Barkhord Kuneem, pg. 38.
 Paighambar Wa Yaaraan, vol. 1, pg. 193; Bihaar al-Anwaar, vol. 10, pg. 43.