Illinois health officials are investigating nine potential cases of severe hepatitis in children under the age of 10 years old, including one case that required a liver transplant, the state’s health department said Wednesday.

That number is up from the three potential cases reported in April.

According to the Illinois Department of Public Health, the cases date back as far as January, with the most recent case experiencing symptoms in May.

Two-thirds of the children tested positive for adenovirus, which could be playing a role in the recent rise in hepatitis cases in children, experts say.

Many adenoviruses are associated with common cold symptoms, such as fever, sore throat and pink eye. Some versions — including adenovirus 41 — can trigger other problems, including inflammation in the stomach and intestines. Adenoviruses previously have been linked to hepatitis in children, but mostly in kids with weakened immune systems.

CDC officials last week said they are now looking into 180 possible cases across the U.S. Most of the children were hospitalized, at least 15 required liver transplants and six died.

More than 20 other countries have reported hundreds more cases in total, though the largest numbers have been in the U.K. and U.S.

Amid recent warnings from the Centers for Disease Control and Prevention and the Illinois Department of Public Health surrounding unusual hepatitis cases in children, what should parents be watching for?

The scope of the problem only started to become clear last month, though disease detectives say they have been working on the mystery for months. It’s been maddeningly difficult to nail a cause down, experts say.

Conventional causes of liver inflammation in otherwise healthy kids — the viruses known as hepatitis A, B, C, D and E — didn’t show up in tests. What’s more, the children came from different places and there seemed to be no common exposures.

What did show up was adenovirus 41. More than half of the U.S. cases have tested positive for adenovirus, of which there are dozens of varieties. In a small number of specimens tested to see what kind of adenovirus was present, adenovirus 41 came up every time.

The fact that adenovirus keeps showing up strengthens the case for it playing a role, but it’s unclear how, Dr. Jay Butler, the CDC’s deputy director for infectious diseases, told The Associated Press.

Recent genetic analysis has turned up no evidence that a single new mutant version of the virus is to blame, said Dr. Umesh Parashar, chief of the CDC group focused on viral gut diseases.

Adenovirus infections are not systematically tracked in the U.S., so it’s not clear if there’s been some recent surge in virus activity. In fact, adenoviruses are so common that researchers aren’t sure what to make of their presence in these cases.

“If we start testing everybody for the adenovirus, they will find so many kids” that have it, said Dr. Heli Bhatt, a pediatric gastroenterologist who treated two Minnesota children with the liver problems.

In Illinois, five patients were reported in the northern part of the state, two in western portions and one each in the central and southern portions. All of the children were hospitalized and one required a liver transplant, but no deaths have so far been reported.

Symptoms of hepatitis — or inflammation of the liver — include fever, fatigue, loss of appetite, nausea, vomiting, abdominal pain, dark urine, light-colored stools, joint pain and jaundice.

“IDPH has been working with Illinois healthcare providers to learn of other suspected potential cases in the state and has asked providers to be on the lookout for symptoms and to report any suspected cases of hepatitis in children of unknown origin to local public health authorities,” the health department said in a release. “IDPH is working with involved local health departments to collect and send available specimens to CDC for further laboratory testing to look more closely at the virus genome and other potential pathogens, such as SARS-CoV-2.”

According to the IDPH, CDC officials currently believe adenovirus may be the cause for these reported cases, but investigators are still learning more – including ruling out other possible causes and identifying other possible contributing factors.

Parents and caregivers are being asked to contact health care providers with any concerns, and to encourage their children to take everyday preventative actions including washing hands often, avoiding people who are sick, and covering coughs or sneezes.