Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Anxiety ?
    ### Response:
    Fear and anxiety are part of life. You may feel anxious before you take a test or walk down a dark street. This kind of anxiety is useful - it can make you more alert or careful. It usually ends soon after you are out of the situation that caused it. But for millions of people in the United States, the anxiety does not go away, and gets worse over time.
    