Depression is a mood disorder characterized by persistent feelings of sadness and loss of interest. It affects how a person feels, thinks and behaves. Common symptoms include feelings of sadness, anger, loss of interest in activities, changes in appetite or sleep, fatigue, concentration problems, and suicidal thoughts. Depression is caused by biological and environmental factors and can become a long-term illness if left untreated. Treatment options include medication, psychotherapy, electroconvulsive therapy or transcranial magnetic stimulation.