Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Leukemia ?
    ### Response:
    Leukemia is a cancer of the blood cells. It is the most common type of blood cancer and affects 10 times as many adults as children. Most people diagnosed with leukemia are over 50 years old. Leukemia Starts in Bone Marrow Leukemia usually begins in the bone marrow, the soft material in the center of most bones where blood cells are formed.
    