Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Fistulas ?
    ### Response:
    A fistula is an abnormal connection between two parts inside of the body. Fistulas may develop between different organs, such as between the esophagus and the windpipe or the bowel and the vagina. They can also develop between two blood vessels, such as between an artery and a vein or between two arteries.
    