Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Muscular Dystrophy ?
    ### Response:
    Muscular dystrophy (MD) is a group of more than 30 inherited diseases. They all cause muscle weakness and muscle loss. Some forms of MD appear in infancy or childhood. Others may not appear until middle age or later. The different types can vary in whom they affect, which muscles they affect, and what the symptoms are.
    