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 that experience it need treatment to get better.

Over 19 million American adults have a depressive disorder. Nearly twice as many women (6.5 percent) as men (3.3 percent) have major depressive disorder. Depression can happen to anyone, regardless of age, race or ethnic group.Depressive disorders may be appearing earlier in life in people born in recent decades compared to the past and often co-occur with anxiety disorders and substance abuse.

People with depressive illnesses do not all experience the same symptoms. The severity, frequency and duration of symptoms will vary depending on the individual and his or her particular illness. Symptoms include:

• Persistent sad, anxious or "empty" feelings
• Feelings of hopelessness and/or pessimism
• Feelings of guilt, worthlessness and/or helplessness
• Irritability, restlessness
• Loss of interest in activities or hobbies once pleasurable, including sex
• Fatigue and decreased energy
• Difficulty concentrating, remembering details and making decisions
• Insomnia, early-morning wakefulness, or excessive sleeping
• Overeating, or appetite loss
• Thoughts of suicide, suicide attempts
• Persistent aches or pains, headaches, cramps or digestive problems that do not ease even with treatment

There is no single known cause of depression. Rather, it likely results from a combination of genetic, biochemical, environmental, and psychological factors.

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

• Major depressive disorder, also called major depression, 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.
• 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.
• 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. It is estimated that 10 to 15 percent of women experience postpartum depression after giving birth.
• 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).

The majority of people with depressive disorders improve when they receive appropriate treatment. The first step to getting treatment is a physical examination by a physician to rule out other possible causes for the symptoms. Next, the physician should conduct a diagnostic evaluation for depression or refer the patient to a mental health professional for this evaluation.

Treatment choice will depend on the patient's diagnosis, severity of symptoms, and preference. A variety of treatments, including medications and short-term psychotherapies (i.e., "talking" therapies), have proven effective for depression. In general, severe depressive illnesses, particulary those that are recurrent, will require a combination of treatment for the best outcome. It usually takes a few weeks of treatment before the full therapeutic effect occurs. Once the person is feeling better, treatment may need to be continued for several months and in some cases, indefinitely-to prevent a relapse.