Depression is an illness that can affect both your mood and your body. It is also referred to as clinical depression or major depressive disorder. People may become depressed for many reasons, including the loss of a loved one, problems with work, a serious illness, or a family history of depression.
Depression may also result from the difficulties of living with other mental and physical conditions. Whatever the reason for depression, doctors have found that people with depression have an imbalance of certain chemicals in the brain. These chemicals (such as serotonin and norepinephrine) act as messengers between the nerve cells and may influence your mood. When these chemicals are low, nerve cells can't communicate as well and the symptoms of depression may occur.
Doctors will determine whether you have depression by asking you about different aspects of your life and how you feel. There are also some written psychological tests that can help with the diagnosis. The symptoms are more than just feeling "bad" or feeling "blue." You may have trouble sleeping, or you may want to sleep all the time. You may not be able to concentrate, have no appetite, or have lost interest in doing a favorite activity or hobby.
Compiled from the following sources: American Psychiatric Association Practice Guideline for Major Depressive Disorder.