Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Polymyositis ?
    ### Response:
    Polymyositis is a type of inflammatory myopathy, which refers to a group of muscle diseases characterized by chronic muscle inflammation and weakness. It involves skeletal muscles (those involved with making movement) on both sides of the body. Although it can affect people of all ages, most cases are seen in adults between the ages of 31 and 60.
    