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, 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.

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. 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).

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, restlessnessLoss of interest in activities or hobbies that were 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

Depression often co–exists with other illnesses. Such illnesses may precede the depression, cause it, and/or be a consequence of it.

It is likely that the mechanics behind the intersection of depression and other illnesses differ for every person and situation. Regardless, these other co–occurring illnesses need to be diagnosed and treated.

Anxiety disorders, such as post–traumatic stress disorder (PTSD), obsessive–compulsive disorder, panic disorder, social phobia and generalized anxiety disorder, often accompany depression. People experiencing PTSD are especially prone to having co-occurring depression. PTSD is a debilitating condition that can result after a person experiences a terrifying event or ordeal, such as a violent assault, a natural disaster, an accident, terrorism or military combat.

People with PTSD often re–live the traumatic event in flashbacks, memories or nightmares. Other symptoms include irritability, anger outbursts, intense guilt, and avoidance of thinking or talking about the traumatic ordeal. In a National Institute of Mental Health (NIMH)–funded study, researchers found that more than 40 percent of people with PTSD also had depression at one-month and four-month intervals after the traumatic event.

Alcohol and other substance abuse or dependence may also co–occur with depression. In fact, research has indicated that the co–existence of mood disorders and substance abuse is pervasive among the U.S. population.

Depression also often co–exists with other serious medical illnesses such as heart disease, stroke, cancer, hiv/aids, diabetes, and Parkinson’s disease. Studies have shown that people who have depression in addition to another serious medical illness tend to have more severe symptoms of both depression and the medical illness, more difficulty adapting to their medical condition, and more medical costs than those who do not have co–existing depression.

Research has yielded increasing evidence that treating the depression can also help improve the outcome of treating the co–occurring illness.

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.  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.

(Adapted from the National Institute of Mental Health.)