In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Allaah [The Most High] said: [فِي قُلُوبِهِمْ مَرَضٌ -In their hearts is a disease]
[Surah Ayah: 10 Soorah Al-Baqarah]
What is intended by disease here is: The disease of doubt, the disease of shubuhaat (i.e. affairs that resemble truth but are falsehood in reality) and the disease of hypocrisy. That is because the heart is exposed to two (types) of diseases that takes it away from well-being and uprightness- the disease of shubuhaat and the disease of lowly destructive desires. Disbelief, hypocrisy, doubts and bidah are all from the disease of shubuhaat. Fornication, love of lewd acts, disobedience and committing them are from the disease of lowly desires.
[Source: Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan’ Abridged and slightly paraphrased]