Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes What I need to know about Crohn's Disease ?
    ### Response:
    Researchers are studying the possible causes of Crohns disease. Your bodys natural defense system, called the immune system, protects you from infection by fighting against bacteria, viruses, and other things that can make you sick. Researchers believe that with Crohns disease, the immune system attacks harmless bacteria and viruses.
    