In Hirschsprung's disease, a section of the gut (bowel) does not work properly. Usually it is a small section towards the very end of the bowel which is affected. It is not known why this is the case, but it seems to be a failure of development of the nerves in one part of the bowel while the baby is in the womb.
This is a companion discussion topic for the original article at https://patient.info/digestive-health/constipation/hirschsprungs-disease