Indeed all praise is due to Allah, we seek His help, and we seek His forgiveness, and we seek refuge in Allah from the evil of our sows and the evil of our actions. Whomsoever Allah guides, none can misguide, and whomsoever Allah misguides, none can guide. I bear witness that there is no deity worthy of worship except for Allah, the One Who has no partner, and I bear witness that Muhammad is His servant and Messenger.
Allah the Exalted said about the hypocrites,
In their hearts is a disease and Allah has increased their disease. [Surah al-Baqarah (2) : 10]
That He may make what is thrown in by Satan a trial for those in whose hearts is a disease and whose hearts are hardened. [Surah al-Hajj (22) : 53]
If the hypocrites, and those in whose hearts is a disease, and those who spread false news among the people of Madinah cease not, We shall certainly let you overpower them, then they will not be able to stay in it as your neighbours but for a little while. [Surah ah-Ahzab (33) : 60]
...and that no doubts may be left for the People of the Book and the believers, and that those in whose hearts is a disease and the disbelievers may say, 'What does Allah intend by this parable?' [Surah al-Muddaththir (74) : 31]
...There has come to you good advice from your Lord, and a healing for that which is in the hearts, a guidance and a mercy for the believers. [Surah Yunus (10) : 57]
And We send down from the Quran that which is a healing and mercy to those who believe, and it increases the wrong-doers in nothing but loss. [Surah al-lsra (17) : 82]
...and heal the breast of a believing people and removes the anger of their hearts ... [Surah Tawbah (9) : 14-15)]
The disease of the body is the opposite of its being sound and in good health, it is a degeneration that occurs in it causing a failure of the natural senses of perception and movement. So with respect to its perception either it goes completely such as blindness or deafness, or it perceives objects incorrectly - such as its perceiving something sweet to be bitter or its hallucinating things that have no reality in the real world. With respect to the failure of its movements then examples of this would be the inability to digest food, or the body's aversion to nourishment that it is need of, or its desire of things that would weaken it, leading to illnesses as a result of these but not leading to death or physical ruin.
Instead these failures would lead to suffering of the actual body either as a result of consuming a wrong quantity of some thing or applying something to the body in the wrong way. As for the first, then it could be consuming too little a quantity of nourishment and therefore the body would require more, of it could be by consuming too much and therefore the body would require it to be removed. As for the second, then it could be like extremely high or low temperatures due to incorrect usage of medicine.
The same is true for the disease of the heart for it is a type of degeneration that occurs in it, causing failure in its perception and desires. So with respect to its perception then this is degenerated by its being presented with doubts upon doubts until it cannot see the truth or it perceives the truth incorrectly. Its desires are degenerated by its hating the truth which would be of benefit to it, and loving the falsehood that would cause it harm. So this is why 'diseases' has sometimes been explained to be doubt and suspicion, as was explained by Mujahid and Qatadah in their commentaries to the verse;
In their hearts is a disease and Allah has increased their disease. [Surah al Baqarah (2) : 10]
and at other times to be the desire to commit fornication as in the case of the verse;
...Lest he in whose heart is a disease be moved with desire. [Surah al-Ahzab (33) : 32]
This is why al-Kharaiti authored a book called, 'The book of the Weakness of the Hearts Meaning their Diseases,' meaning by 'diseases' here - the diseases of desire.
The sick person is harmed by things that the healthy person is not, so slight heat, cold, exertion or other such things will harm him due to his inability to endure them in his weakened state. Sickness, in general, weakens the one afflicted by making his endurance weak and unable to sustain what he would have been able to sustain in a strong state. So a healthy state is preserved by remaining healthy and is removed by the opposite, and the sickness is made more severe by the presence of conditions similar to those that led to the sickness in the first place and removed by the opposite. Therefore, if a sick person is afflicted by something similar to that which led him to being sick in the first place, then he increases in illness and his endurance becomes weaker, until maybe he dies. But if he is affected by something that will increase his strength and weaken the illness then the opposite will occur.
The disease of the heart is a pain that occurs in the heart such as the anger felt towards an opponent who overcomes you, for this hurts the heart.
Allah, the Exalted said,
...and heal the breast of a believing people and removes the anger of their hearts... [Surah Tawbah (9) : 14-15]
So the healing for them was by removing the suffering that had occurred in their hearts, and it is said: 'So and so has healed his anger.' In the case of retaliation it is said: 'The close relatives of the killed sought healing,' meaning healing of their grief, anger and sorrow - all of these being sufferings that occur in oneself. Likewise doubt and ignorance cause pain to the heart. The Prophet said,
...Could they not have asked if they did not know? Indeed the cure for ignorance is to ask... [Sunan Abu Dawood]
And the one who has doubt in something he has taken on board, causes harm to his heart until he attains knowledge and certainty. Hence it is said to a scholar when he answers in a way that clarifies the truth: 'you have healed me with the answer.'
Concerning the Ailments of the Hearts and their Cures
Between Sickness And Death
The Quran Is A Cure For The Hearts
Righteous Actions Are A Cure For The Heart
Leaving Indecent Actions Are A Cure For The Heart
The Effect Of Sins Upon The Purity Of The Heart
Types Of Zulm
The Life Of The Heart
The State Of The Dead Heart
The Need For Beneficial Knowledge
The Reality Of The Life Of The Heart
Envy Is A Sickness Of The Heart
The Types Of Envy (hasad)
Between Hasad And Ghubta
The Competition Between Abu Bakr And Umar
Moses Displays Ghubta
Whoevers Ranking Becomes Lofty, He Is Secured From Ghubta
The Cure For Jealousy
The Causes For Jealousy
Between Jealousy And Miserliness
The Disease Of Desires And Passionate Love
Between Jealousy And Desires
The Reality Of Passionate Love (ishk)
The Natural Inclination Of The Heart Is To Love Of Allah
Preventative Measures From Passionate Love
Some Cures For The Heart
Appendix One The Types Of Hearts
Glossary Of Arabic Terms