Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Depression ?
    ### Response:
    Depression is a serious medical illness. It's more than just a feeling of being sad or "blue" for a few days. If you are one of the more than 19 million teens and adults in the United States who have depression, the feelings do not go away. They persist and interfere with your everyday life.
    