What is Depressive Disorder? Understanding the Symptoms and Causes Depressive Disorder, also known as major depressive disorder (MDD), is a common but serious medical condition that affects how a person feels, thinks, and behaves. It causes persistent feelings of sadness and loss of interest, which can lead to various emotional and physical problems. People with depression may struggle to carry out everyday activities and can experience feelings of worthlessness or guilt. Understanding the symptoms and causes of depressive disorder is essential for identifying it early and seeking effective treatment.