Diseases of the heart are more severe than diseases of the body, because the eventual end of bodily diseases is death—and death is an inevitable outcome. Allah the Most High said:

كُلُّ نَفۡسٖ ذَآئِقَةُ ٱلۡمَوۡتِۖ ثُمَّ إِلَيۡنَا تُرۡجَعُونَ

Everyone shall taste the death. Then unto Us you shall be returned. Al-‘Ankabut 29:57

However, diseases of the heart are more dangerous, because if they persist, then indeed the heart dies—meaning it becomes completely ruined. The possessor of such a heart may become a disbeliever, or deviated, or among the open sinners. Therefore, diseases of the heart are more severe and more dangerous for people than bodily diseases. There is no cure for diseases of the heart except through the Noble Qur’an, which Allah sent as a cure for mankind. He the Most High said:

وَنُنَزِّلُ مِنَ ٱلۡقُرۡءَانِ مَا هُوَ شِفَآءٞ وَرَحۡمَةٞ لِّلۡمُؤۡمِنِينَ وَلَا يَزِيدُ ٱلظَّٰلِمِينَ إِلَّا خَسَارٗا

And We send down from the Quran that which is a healing and a mercy to those who believe (in Islamic Monotheism and act on it), and it increases the Zalimun (polytheists and wrong-doers) nothing but loss. Al-Isrā’ 17:82

Muhaadaraat fee al-Aqeedah wa al-Dawah v.1 pg.280