Everyone occasionally feels blue or sad, but these feelings are usually fleeting and pass within a couple of days. However, when the down times last for weeks or months at a time or keep you from living "normal," you may be suffering from depression.

What it is
Depression is a medical illness that involves the body, mood, and thoughts.  It affects the way you eat and sleep, the way you feel about yourself, and the way you think about things.

Causes
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.