The mysterious outbreak of pediatric hepatitis in very little kids has turned deadly in the US, the Centers for Disease Control and Prevention confirmed on Friday.

Dr. Jay Butler, deputy director for infectious diseases at the CDC, told reporters on a call that the agency is investigating 109 unusual hepatitis cases across 24 states, plus Puerto Rico.

At least five of those cases have been deadly, and 14% required liver transplants, demonstrating how serious these cases can be, though they are still quite rare.

The CDC says it's tracking and investigating cases that have happened "over the past seven months" in children younger than 10 years old across:

More than half of the children in the US had an adenovirus infection when they contracted their liver inflammation, suggesting there could be some kind of viral trigger for this hepatitis, but it's still unclear exactly what is causing these cases. On Friday, the UK Health Security Agency released a technical briefing, saying the "leading hypotheses remain those which involve adenovirus."

A majority of the 163 mysterious hepatitis cases under investigation in the UK have tested positive for adenovirus too:

Still, the UK HSA added, "we continue to investigate the potential role of SARS-CoV-2 [the virus that causes COVID-19], and to work on ruling out any toxicological component."

The CDC, like the UK HSA said it's possible environmental factors, medicines, or other non-adenovirus infections (like COVID-19 infections) could be playing a role.

"It's still early days in terms of pinpointing the cause," Dr. Umesh Parashar, the CDC's chief of viral diseases, said on the call.

But, "COVID-19 vaccination is not the cause of these illnesses," Butler said.

That's because most of these kids are still too young to be vaccinated against COVID, with a median age of just two years old.

In Alabama where the first nine confirmed cases were identified in the US, no child had a documented history of COVID-19 infection, and none were sick with COVID during their hospitalization either. Most of the children who've been diagnosed with this form of dangerous but rare hepatitis over the past seven months have made a full recovery, the CDC said.

Dr. Mike Leonis, a pediatric hepatologist at Cleveland Clinic Children's who's been practicing for two decades, told Insider that the number of patients testing positive for adenovirus in this outbreak "seems high to me," but he also thinks it's "not time to get super panicked about this."

"I'm not hearing anything to suggest that it spreads rapidly and it's really, really severe," he said.

Often, Leonis explained, doctors aren't ultimately able to figure out a precise cause for many cases of severe hepatitis in kids.

"Many, many times, the child that I'm looking at in the intensive care unit who we are considering for a liver transplant had two siblings who got a cold along with them, and the other two are fine," he said. "Is it because they had a second virus on board? Is it because they had a different combination of genes that made their immune system behave differently? Is it because they were on a different medication that kind of primed their liver to be more sensitive? We really don't know."

The CDC is urging doctors, parents, and caregivers to watch out for:

These are all signs that a child may be coming down with hepatitis.

In terms of prevention, basic hygiene measures are what's recommended. Adenoviruses, like other viruses, spread through the air from person to person, and also via the fecal-oral route. Washing hands before eating or handling food, and after changing diapers, going to the bathroom, or touching a snotty tissue are all important. Masks help too.

"It's so easy to forget," Leonis said. "I can imagine a busy daycare center, you've got three babies that need their diapers changed. So maybe you just think, 'oh, my hands are clean, because I didn't get 'em dirty on baby number one.' And you move on to baby number two. No, no, no."

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