Wisconsin health officials are investigating four cases of children with significant unexplained liver damage.
This includes one child who needed a liver transplant and one who died.
The Wisconsin Department of Health Services (DHS) issued a health alert Wednesday due to "a recent increase in cases of acute hepatitis and adenovirus infection in children."
Doctors are asked to be on the lookout for these unusual cases and report them.
Wisconsin is the fourth state to investigate the illnesses, which does not seem to be caused by any of the usual suspects, such as Hepatitis A,B, C or D.
From November to February, clinicians in Alabama identified nine pediatric patients with significant liver damage. Health officials say all children were previously healthy. Three children had acute liver failure and also tested positive for adenovirus. All five of the nine had adenovirus type 41 infection. Two of the children required a liver transplant. No patients died.