Feeling depressed is a common human experience, occurring most often as a normal response to external events or personal loss. However, a major depressive episode, whether of unknown origin or a part of a defined neurological disorder, is a pathological condition. Depression is characterized as one of the major affective disorders, all of which share a primary dysregulation in mood. It is diagnosed not only by the presence of persistent negative mood but also by pronounced and unrelenting disturbances in attention, motivation, motor and mental speed, sleep, appetite, and libido. These symptoms occur in combination with sustained anhedonia (that is, the inability to experience pleasure), excessive or inappropriate guilt, recurrent thoughts of death with suicidal ideations, and in some cases suicide attempts.