Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes What I need to know about Hirschsprung Disease ?
    ### Response:
    People with HD have constipation because they lack nerve cells in a part or all of the large intestine. The nerve cells signal muscles in the large intestine to push stool toward the anus. Without a signal to push stool along, stool will remain in the large intestine.
    