Depression is a serious medical illness that involves the brain. It's more than just a feeling of being "down in the dumps" or "blue" for a few days. If you are one of the more than 20 million people in the United States who have depression, the feelings do not go away. They persist and interfere with your everyday life. There are a variety of causes, including genetic, environmental, psychological, and biochemical factors. Depression usually starts between the ages of 15 and 30, and is much more common in women. Women can also get postpartum depression after the birth of a baby. Some people get seasonal affective disorder in the winter.