One child has died and more than a dozen have undergone liver transplants as a result of a mysterious outbreak of severe acute hepatitis that’s sickened children in the U.K., the U.S. and 10 other countries, the World Health Organization said.

Health authorities are trying to determine the source of the liver-inflaming disease that’s afflicted at least 169 children, ages 1 month to 16 years, as of April 21, the WHO said in a statement Saturday. Typical causes of viral hepatitis have been excluded.

The United Nations agency was notified on April 5 of 10 cases among previously healthy children across central Scotland with jaundice, diarrhea, vomiting and abdominal pain. Three days later, 74 cases had been identified in the U.K.