Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) GERD ?
    ### Response:
    Your esophagus is the tube that carries food from your mouth to your stomach. Gastroesophageal reflux disease (GERD) happens when a muscle at the end of your esophagus does not close properly. This allows stomach contents to leak back, or reflux, into the esophagus and irritate it.
    