Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Chronic fatigue syndrome ?
    ### Response:
    Chronic fatigue syndrome, also known as systemic exertion intolerance disease, is a condition that causes extreme, long-lasting fatigue which can limit the ability to participate in ordinary, daily activities.  It generally occurs in young adults between the ages of 20 and 40 and is twice as common in women.
    