Symptoms include fever, fatigue, nausea, vomiting, abdominal pain, joint pain and jaundice.
U.S. health officials said as many as 109 cases of a mysterious and severe liver disease in children are under investigation.
The Centers for Disease Control and Prevention put out a new alert for doctors to be on the lookout for surprising cases of hepatitis. About two dozen states and territories reported suspected cases, which date back to October in children under 10.
ABC Chief Medical Correspondent Dr. Jen Ashton detailed on "Good Morning America" Wednesday what experts know about this outbreak and symptoms parents should monitor.
Note: This interview was edited for clarity.
Ashton: The CDC put out an alert for health care providers two weeks ago. They're tracking these cases back to October, and they do expect this number to grow.
Hepatitis is a catch-all term for acute liver injury. In the pediatric population right now, experts think that most of these cases are associated with an adenovirus 41, which is a common gastrointestinal virus, but not all of them.
Ninety percent of the children affected have required hospitalizations. There have tragically been five deaths, and 14% of cases have required liver transplantation.
This is an evolving situation that right now, experts are in the stages of accumulating data and observation.
Ashton: Experts don't think so. Remember, some of these children coincidentally have had COVID-19.
I just spoke to CDC Director Dr. Rochelle Walensky. She wanted me to emphasize that the majority of cases have been in children ages 2 to 5. These children, as we all know, are not eligible for the COVID vaccine, so this outbreak has nothing to do with the vaccine.
Right now, researchers can see cases of hepatitis following any virus. It's still unclear what's causing these cases.
Ashton: They are, but not this strain of hepatitis. There's hepatitis A, B, C and D. These cases of hepatitis are none of those, so it's mystifying to public health officials at this point.
Right now, the CDC is really increasing the alert to parents to be on the lookout for signs and symptoms.
Ashton: Symptoms include fever, fatigue, nausea, vomiting, abdominal pain, joint pain and jaundice -- a yellowing of the white part of the eyes or the skin or a change in the color of urine or stool.
This obviously should drive the parents to alert their pediatricians as quickly as possible. I want to emphasize even though the cases are growing, this is still very uncommon.
Awareness is key.
The Associated Press contributed to this report.