The U.S. Centers for Disease Control and Prevention is investigating mysterious cases of hepatitis among young children.

In 109 cases since October being investigated, five children died and several more needed a liver transplant. About half of the children had confirmed cases of an adenovirus, a common class of viruses that can cause congestion and flulike symptoms.

Doctors don’t yet know the cause of these cases but are researching whether there may be a link between hepatitis and adenovirus, as well as other potential contributing factors.

The vast majority of children who developed hepatitis recovered. Still, severe hepatitis is rare among children, which is why doctors and the CDC have urged parents to be alert.

Here’s what to know:

What is hepatitis?

Hepatitis is the medical term to describe liver inflammation. Hepatitis occurs on a spectrum, meaning patients can experience minor or severe inflammation, said Salwa Sulieman, a pediatric infectious disease specialist at Nemours Children’s Health.

What causes hepatitis?

Hepatitis has infectious, meaning it stems from a virus, and non-infectious causes.

Hepatitis viruses A, B, and C are the most common causes of viral hepatitis in the U.S.:

  • Hepatitis A causes acute illness, such as vomiting and diarrhea, and typically resolves within a few weeks. It is spread by ingesting fecal matter from an infected person, for instance through food or water. Though hepatitis is uncommon among children, this is the type they are most likely to be exposed to, Sulieman said.

  • Hepatitis B can cause long-term health problems and is a leading cause of liver cancer. It is spread through blood or other bodily fluids from an infected person.

  • Hepatitis C is spread through blood contact with an infected person, for instance by sharing needles or syringes. It can be passed from an infected mother to an infant at birth.

Other viruses can also cause liver inflammation. Doctors are looking into whether adenovirus, type 40, may be linked to the 109 cases under investigation. They are also exploring the possibility that SARS-CoV-2, the virus that causes COVID-19, may weaken children’s immune systems and make them more vulnerable to an adenovirus infection that could lead to hepatitis, though this was ruled out in some of the first U.S.-reported cases in Alabama.

The most common types of non-infectious hepatitis are overdoses of toxins or medications, such as Tylenol.

What are the symptoms of hepatitis?

Abdominal pain, fatigue, nausea, and jaundice — yellowing of the skin and eyes — are all symptoms of hepatitis. Parents who notice these symptoms, especially jaundice, in their children should seek medical attention immediately, Sulieman said.

What’s causing severe hepatitis among young children?

We don’t know yet. The most common infectious causes — hepatitis A, B, and C — have not been present in the cases the CDC is investigating. Adenovirus was confirmed among about half of the cases, but doctors don’t know yet whether an adenovirus contributed to those children developing hepatitis.

Have there been any cases locally?

The Pennsylvania Department of Health has not confirmed any cases but has notified the CDC it is investigating multiple cases for a possible connection, said Mark O’Neill, a spokesperson for the department.

The New Jersey Department of Health is not aware of any cases, according to department spokesperson Nancy Kearney.

Philadelphia Department of Public Health spokesperson Matt Rankin said it is “participating in the CDC’s investigation,” but declined to say whether the city had reported any cases.

Is hepatitis common among children?

No. Hepatitis viruses are rare among children in the United States, Sulieman said. Anyone can develop liver inflammation when they have a virus or infection, so it’s possible for children to get hepatitis. However, when hepatitis does occur in children, it is unusual for them to have such extreme cases that they would need a liver transplant, she said.

How is hepatitis treated?

Vaccinesrecommended for infants and children — are the most effective way to prevent hepatitis A and B infections. There is no cure for hepatitis, but medications can help reduce symptoms and limit liver damage among people who develop chronic, long-term hepatitis.