Depression is a serious mental health concern and a leading cause of disability worldwide. Approximately 7% of the population in the United States experiences major depression every year. There are higher rates of depression among young adults and females.

Course and chronicity of disease vary by individual – some may have a single episode of depression, while others may have multiple episodes throughout their lifetime. Episode duration, symptom severity, remission duration, and comorbid conditions including anxiety, psychosis, posttraumatic stress disorder, and personality disorders all contribute to determining the likelihood of recovery and recurrence of depression. Adverse childhood experiences and relatives with mental health conditions are also strong risk factors for depression.

Depression is a heterogeneous disease. Generally, patients must have had depressed mood or lack of interest in activities for 2 or more weeks in addition to other symptoms like sleep disturbance, appetite change, impaired attention, fatigue, or suicidal thoughts to meet criteria for major depressive disorder. Atypical presentations of depression are common among the pediatric and geriatric populations. Adolescents may present with more irritability than depressed mood, and they are more likely than adults to present with appetite change, weight change, loss of energy, and sleep disturbance. Elderly patients can present with memory loss or confusion.

Patients with this disorder may have an increased risk of various general medical conditions. Early onset of depression and depression with psychotic behavior are among the risk factors for bipolar disorder.