Public health officials in Europe and the United States are investigating dozens of puzzling cases of severe hepatitis in young children. Hepatitis is an inflammation of the liver. The cause is often a virus, but the viruses that commonly cause the illness — hepatitis A, B, C, D and E — have been ruled out in the cases in question, leaving doctors searching for the culprit. In several cases, the illness was so severe that the children needed a liver transplant. No deaths have been reported. The World Health Organization on Friday said it was investigating 74 cases of severe...