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Major depression is a disorder characterized by an altered, usually profoundly sad mood and a loss of interest or pleasure in activities that were once enjoyable (anhedonia). This disorder also impairs the patient's ability to function, such as in school or at work.

Depression may be classified as mild, moderate or severe in intensity. Episodes of major depression may occur suddenly or gradually over time. Patients may experience a single episode or recurrent episodes. An episode may last from a few weeks to many years, but most last between six and nine months.

In most cases, symptoms of major depression disappear completely or almost completely at the end of an episode. However, sometimes only some of the symptoms disappear or the episode does not seem to end at all. A mild depression that lasts for at least two years and does not quite add up to a major depression is known as chronic depression or is sometimes referred to as dysthymia.

Although major depression varies widely between patients, the symptoms in a given patient usually remain consistent from one episode to the next. Multiple episodes tend to increase the degree of impairment. Further, the risk of future episodes increases with each additional episode. According to the American Psychiatric Association (APA):

• Patients who experience one episode of major depression have a 60 percent risk of experiencing a second episode
  
  
• Patients who experience two episodes have a 70 percent risk of experiencing a third episode
  
  
• Patients who experience three episodes have a 90 percent risk of experiencing a fourth episode

Although depression can affect individuals of all ages, including children, it tends to begin during the mid-20s or early 30s.

Major depression affects about 14.8 million American adults each year, according to the National Institute of Mental Health (NIMH). It is diagnosed in women nearly twice as often as men.

In addition, about 2 percent of children between the ages of 6 and 12 years suffer from major depression at any given time. This rate increases to 4 percent at puberty. In all, around 20 percent of all individuals will have at least one episode of major depression before they reach adulthood.

Major depression often follows severe mental stress (e.g., death of a loved one, divorce). It may also affect women after childbirth (postpartum depression). Another type of depression occurs during a particular season, usually in winter with symptoms going away during summer (seasonal affective disorder).

Suicide is a risk with major depression when other risk factors for suicide exist. As many as 15 percent of patients with major depression die by suicide, according to the APA, and this number is even greater among elderly white men and teenagers. Multiple episodes of depression greatly increase the risk of suicidal thoughts and attempts.

Suicide risk is of particular concern with children and adolescents suffering from major depression. Suicide rates among young people have nearly tripled since 1960. Almost 5,000 individuals between the ages of 15 and 24 years kill themselves each year, according to the National Mental Health Association (NMHA). This makes suicide the third-leading cause of death in adolescents and the second-leading cause of death among college-age youth. It is believed that many of these suicide victims suffered from untreated depression.

Dysthymia is a chronic but less severe form of depression in which depression symptoms last at least two years. Patients who suffer from dysthymia are usually able to function adequately. There is a close relationship between major depression and dysthymia. For instance, many patients with dysthymia will eventually develop major depression, and patients with major depression may eventually develop dysthymia. Patients with dysthymia may develop a major depressive episode that is referred to as double depression.

In addition, about 5 to 10 percent of patients who have had a major depressive episode will eventually have a manic episode (characterized by an elevated mood and excessive energy and optimism) and be diagnosed with bipolar disorder, according to the APA.

Major depression is commonly associated with other conditions, including eating disorders (e.g., anorexia nervosa, bulimia nervosa), anxiety disorders (e.g., obsessive-compulsive disorder) and borderline personality disorder. In addition, about one in three depressed patients also suffers from some form of substance abuse or dependence, according to the NMHA. Depressed patients may also experience more pain and general physical illnesses and are at a higher risk of developing chronic conditions such as coronary artery disease.

Major depression affects those close to the patient, as well. Adult first-degree biological relatives (e.g., brothers, sisters) may have an increased risk of alcoholism. The children of patients with major depression may have an increased risk of an anxiety disorder or attention deficit hyperactivity disorder.

Major depression is one of the leading causes of disability in the United States and internationally, according to the World Health Organization.