Gastroesophageal reflux disease (GERD) is one of the most common digestive disorders in the world. It happens when acid comes up from the stomach, which is acid-resistant, into the esophagus, which is less acid-resistant. Dr. James East, a gastroenterologist at Mayo Clinic Healthcare in London, says GERD may be common, but there can be potentially severe complications if it’s ongoing and left untreated.