Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Myositis ?
    ### Response:
    Myositis means inflammation of the muscles that you use to move your body. An injury, infection, or autoimmune disease can cause it. Two specific kinds are polymyositis and dermatomyositis. Polymyositis causes muscle weakness, usually in the muscles closest to the trunk of your body.
    