Everyone occasionally feels blue or sad, but these feelings are usually fleeting and pass within a couple of days. When a person has a depressive disorder, it interferes with daily life, normal functioning, and causes pain for both the person with the disorder and those who care about him or her. Depression is a common but serious illness, and most who experience it need treatment to get better.
Many people with a depressive illness never seek treatment. But the vast majority, even those with the most severe depression, can get better with treatment. Intensive research into the illness has resulted in the development of medications, psychotherapies, and other methods to treat people with this disabling disorder.

There are several forms of depressive disorders. The most common are major depressive disorder and dysthymic disorder.

Major depressive disorder is characterized by a combination of symptoms that interfere with a person's ability to work, sleep, study, eat, and enjoy once-pleasurable activities. Major depression is disabling and prevents a person from functioning normally. An episode of major depression may occur only once in a person's lifetime, but more often, it recurs throughout a person's life.
Major depressive disorder, also known as major depression, unipolar depression, clinical depression, or simply depression, is a mental disorder characterized by a pervasive low mood, loss of interest in usual activities and diminished ability to experience pleasure. The diagnosis is made if a person has suffered one or more major depressive episodes. The onset is usually in early- to mid-adulthood. Diagnosis is based on the patient's self-reported experiences and observed behavior. There is no laboratory test for major depression, although physicians often test for physical conditions that may cause similar symptoms before arriving at a diagnosis. The course varies widely: it can be a once-in-a-lifetime event or have multiple recurrences; it can appear either gradually or suddenly; and can either last for a few months or be a life-long disorder.

Dysthymic disorder, also called dysthymia, is characterized by long term (two years or longer) but less severe symptoms that may not disable a person but can prevent one from functioning normally or feeling well. People with dysthymia may also experience one or more episodes of major depression during their lifetimes.
The symptoms of dysthymia are similar to those of major depression, though they tend to be less intense. In both conditions, a person can have a low or irritable mood, lack of interest in things most people find enjoyable, and a loss of energy. Appetite and weight can be increased or decreased. The person may sleep too much or have trouble sleeping. He or she may have difficulty concentrating. The person may be indecisive and pessimistic and have a poor self-image. The symptoms can grow into a full blown episode of major depression. This situation is sometimes called "double depression"[2] because the intense episode exists with the usual feelings of low mood. People with dysthymia have a greater-than-average chance of developing major depression. While major depression often occurs in episodes, dysthymia is more constant, lasting for long periods, sometimes beginning in childhood, as a result a person with dysthymia tends to believe that depression is a part of his or her character. The person with dysthymia may not even think to talk about this depression with doctors, family members or friends. Dysthymia, like major depression, tends to run in families. It is two to three times more common in women than in men. Some people with dysthymia experienced a major loss in childhood, such as death of a parent. Others describe being under chronic stress. It is often difficult to tell whether people with dysthymia are under more stress than other people or if the dysthymia causes them to perceive more stress than others.

Some forms of depressive disorder exhibit slightly different characteristics than those described above, or they may develop under unique circumstances. However, not all scientists agree on how to characterize and define these forms of depression. They include:

Psychotic depression, which occurs when a severe depressive illness is accompanied by some form of psychosis, such as a break with reality, hallucinations, and delusions.

Postpartum depression, which is diagnosed if a new mother develops a major depressive episode within one month after delivery. Women are particularly vulnerable to depression after giving birth, when hormonal and physical changes and the new responsibility of caring for a newborn can be overwhelming. Many new mothers experience a brief episode of mild mood changes known as the "baby blues," but some will suffer from postpartum depression, a much more serious condition that requires active treatment and emotional support for the new mother. One study found that postpartum women are at an increased risk for several mental disorders, including depression, for several months after childbirth.8

Some studies suggest that women who experience postpartum depression often have had prior depressive episodes. Some experience it during their pregnancies, but it often goes undetected. Research suggests that visits to the doctor may be good opportunities for screening for depression both during pregnancy and in the postpartum period.

Seasonal affective disorder (SAD), which is characterized by the onset of a depressive illness during the winter months, when there is less natural sunlight. The depression generally lifts during spring and summer. SAD may be effectively treated with light therapy, but nearly half of those with SAD do not respond to light therapy alone. Antidepressant medication and psychotherapy can reduce SAD symptoms, either alone or in combination with light therapy.

Bipolar disorder, also called manic-depressive illness, is not as common as major depression or dysthymia. Bipolar disorder is characterized by cycling mood changes-from extreme highs (e.g., mania) to extreme lows (e.g., depression).



There is no single known cause of depression. Rather, it likely results from a combination of genetic, biochemical, environmental, and psychological factors.
Research indicates that depressive illnesses are disorders of the brain. Brain-imaging technologies, such as magnetic resonance imaging (MRI), have shown that the brains of people who have depression look different than those of people without depression. The parts of the brain responsible for regulating mood, thinking, sleep, appetite and behavior appear to function abnormally. In addition, important neurotransmitters, chemicals that brain cells use to communicate, appear to be out of balance. But these images do not reveal why the depression has occurred.
Some types of depression tend to run in families, suggesting a genetic link. However, depression can occur in people without family histories of depression as well.9 Genetics research indicates that risk for depression results from the influence of multiple genes acting together with environmental or other factors.
In addition, trauma, loss of a loved one, a difficult relationship, or any stressful situation may trigger a depressive episode. Subsequent depressive episodes may occur with or without an obvious trigger.