( CNN ) -- Britain 's Queen Elizabeth II was discharged Monday from a London hospital , where she was treated for symptoms of gastroenteritis . The 86-year-old monarch had been taken to King Edward VII 's Hospital on Sunday morning `` as a precautionary measure , '' a Buckingham Palace spokesman said , speaking with customary anonymity . The queen smiled as she left the hospital Monday . It was the first time the queen had been hospitalized in a decade . Her official engagements for the week were postponed or canceled Sunday . Elizabeth had already canceled a planned trip to Wales on Saturday after showing symptoms of the illness , Buckingham Palace said . Five things to know about gastroenteritis . Gastroenteritis is an inflammation of the lining of the stomach and intestines . Though commonly known as `` stomach flu , '' it is not caused by the influenza virus , according to the U.S. Centers for Disease Control and Prevention . The illness is usually caused by a virus and passed from person to person , said Dr. Corey Siegel , a gastroenterologist and professor at Dartmouth College 's Geisel School of Medicine . But it can also be caused by bacteria , either foodborne or personally transmitted , he said . The bug usually has to run its course . But doctors often give hospitalized elderly patients intravenous fluids to prevent dehydration , which can lead to kidney problems , Siegel said . Queen 's illness : Concern behind the castle walls . Elizabeth celebrated her 60th anniversary on the throne in 2012 and turns 87 in April . She was last hospitalized in 2003 for knee surgery . Her husband , the 92-year-old Prince Philip , was hospitalized three times between December 2011 and August 2012 -- once for treatment of a blocked coronary artery and twice for a bladder infection . Get the latest news from CNN.com . CNN 's Richard Quest , Elizabeth Cohen , Per Nyberg and Max Foster contributed to this report .
NEW : London 's Elizabeth II is now out of Buckingham Palace Hospital . Gastroenteritis is an inflammation of the lining of the stomach and intestines . The illness can cause dehydration and kidney problems in the elderly , a doctor says . Queen Elizabeth II was hospitalized `` as a precautionary measure , '' Prince Philip says .