Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Crohn's Disease ?
    ### Response:
    The exact cause of Crohn's disease is unknown. Researchers believe the following factors may play a role in causing Crohn's disease:
                
- autoimmune reaction  - genes  - environment
                
Autoimmune reaction. Scientists believe one cause of Crohn's disease may be an autoimmune reactionwhen a person's immune system attacks healthy cells in the body by mistake.
    