Eleven countries, including the U.S., have reported a total of 169 cases of severe acute hepatitis in healthy children ages 1 month to 16-years-old, with the largest outbreak in the U.K.

At least 17 children have required liver transplants and one patient has died, according to the WHO.

The World Health Organization and the Centers for Disease Control and Prevention believe the outbreak might be linked to adenovirus, though investigations are ongoing.