Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Parkinson disease ?
    ### Response:
    Parkinson disease is a progressive disorder of the nervous system. The disorder affects several regions of the brain, especially an area called the substantia nigra that controls balance and movement.  Often the first symptom of Parkinson disease is trembling or shaking (tremor) of a limb, especially when the body is at rest.
    