" The unseen aspect of the heart contains a bad seed that has the potential of becoming like a cancer that can metastasize and overtake the heart. The bacterium responsible for tuberculosis, for example, lives latent in the lungs of millions of people. When its carriers age or succumb to another disease that weakens their immune system, tuberculosis may start to emerge. This analogy illustrates that there is a dormant element in the human heart that, if nurtured and allowed to grow, can damage the soul and eventually destroy it. "

― Hamza Yusuf , Purification of the Heart: Signs, Symptoms and Cures of the Spiritual Diseases of the Heart