Sickness of the heart is of two kinds: sickness of uncertainty and doubt, and sickness of desire and temptation.
The first is known as shubuhāt or obfuscations, diseases that relate to impaired understanding. For instance, if somebody is fearful that Allah SWT will not provide for him or her, this is considered a disease of the heart because a sound heart has knowledge and trust, not doubt and anxiety.
Shubuhāt alludes to aspects closely connected to the heart: the soul, the ego, Satan’s whispering and instigation, caprice, and the ardent love of this ephemeral world.
The second category of disease concerns the base desires of self and is called shahawāt. This relates to our desires exceeding their natural state, as when people live merely to satisfy these urges and are led by them Sickness of the heart can be avoided if we are able to conquer arrogance with humility and intelligence, uncontrolled ambition with conscience and selflessness, and envy with compassion and patience.
Submitting to the will of Allah SWT and living according to the Qurʾān and Sunnah will enable us to avoid succumbing to nafs al-ammārah (excess of passion with negative traits of envy, jealousy and lust etc.) and toward nafs al-lawwāmah, wherein reason controls the passions with Allah SWT consciousness.