Some cases were so serious, they resulted in transplants and one reported death.
READ MORE | 1 child has died in mystery liver disease outbreak, World Health Organization says
Pediatric infectious disease specialist Dr. Santosh Nadipuram with Cedars-Sinai Medical Center said the first spike in cases was reported in the United Kingdom.
"By the 8th of April, they found 74 cases in total, and these were all in children who are about 11 months, all the way up to 10 years. But most of them are under five years old," he said.
The World Health Organization soon learned of 169 cases throughout Europe, with nine in Alabama.
Of those nine, two needed liver transplants.
Globally, 74 of the children had been infected with a common cold adenovirus. Many with a specific one identified as adenovirus 41.
"The ones in Alabama had adenovirus, and that was what seems to be that common thread," Nadipuram said.
Adenoviruses are not new. They're known to cause pink eye, respiratory and gastrointestinal infections.
Doctors said adenovirus 41 can cause hepatitis, but only in children with compromised immune systems. All the kids affected were reported to be healthy. So did the virus change? Researchers said the investigation is just beginning.
"It's our best candidate right now, but we are not 100% sure that this is the cause behind these uptick in cases," said Nadipurm.
Signs and symptoms to look out for include yellow skin and eyes, abdominal pain, nausea and vomiting, dark urine and pale bowel movements.
Nadipurm said the best way to avoid getting an adenovirus is with good hand hygiene.
"Soap and water is really best if possible, especially before eating and before doing anything where you're touching your face," he said.
Although some of the patients had both COVID-19 and an adenovirus, experts said COVID does not appear to be the cause.