Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Chronic Fatigue Syndrome (CFS) ?
    ### Response:
    Chronic fatigue syndrome, or CFS, is a devastating and complex disorder. People with CFS have overwhelming fatigue and a host of other symptoms that are not improved by bed rest and that can get worse after physical activity or mental exertion. They often function at a substantially lower level of activity than they were capable of before they became ill.
    