Regarding the hypocrites, Allah ta’ala mentions in Surah Baqarah: Verse 10
فِي قُلُوبِهِمْ مَرَضٌ – In their hearts there is a malady…
Now, illness or disease, in the general medical sense, is a state in which a man has lost the balanced proportion of the elements within him necessary to keep him healthy, so that his body can no longer function properly, which may finally lead to his total destruction.
In the terminology of the Holy Qur’an and the Hadith, the word ‘disease’ is also applied to certain mental or psychic states (we are using the two terms in the original and more comprehensive sense) which hinder man from attaining any degree of spiritual perfection, for they gradually deprive him of the ability to perform good deeds, and even of ordinary human decency, till he meets with his spiritual death.
The great spiritual master, Junaid of Baghdad has said,
Just as the diseases of the body arise from an imbalance among the four humours, the diseases of the heart arise from a surrender to one’s physical desires.
From Ma’ariful Qur’an