I see it every week: A person who is sad, having difficulty functioning or concentrating, crying frequently. Some have anxiety, other feel achy all over, still others sleep too much or too little. Most say that they feel utterly alone. This is depression.

Depression affects around 19 million Americans at any one time. And treatment does help reduce symptoms and in most cases cause remission of the disease.

How to determine if the sad feelings are true depression or just the blues is a real...