Health officials are still looking for answers related to mysterious cases of severe liver damage in hundreds of young children around the world.

The best available evidence points to a common stomach virus not known to cause liver problems in otherwise healthy children. That virus was found in the blood of sick children, but it has not been found in their diseased livers.

Eric Kremer is a virus researcher at the Institute of Molecular Genetics of Montpellier in France. He said, "There's a lot of things that don't make sense."

The U.S. Centers for Disease Control and Prevention and health officials around the world are trying to find out what is going on.

The illnesses are considered rare. CDC officials recently said they are now looking into 180 possible cases across the United States. Most of the children were hospitalized. At least 15 required new livers and six died.

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

Some signs of liver inflammation include fever, tiredness, loss of interest in eating and nausea. Others include stomach pain, joint pain and jaundice – a condition that causes a person's skin to turn yellow.

Disease experts say they have been working on the mystery illness for months but have struggled to find an exact cause.

The usual causes of liver inflammation in otherwise healthy children come from viruses known as hepatitis A, B, C, D and E. But none of those viruses appeared in tests. The children came from different places and there seemed to be no common means of virus contact.

What did show up in tests was a virus known as adenovirus 41. More than half of the U.S. cases have tested positive for adenovirus. In a small number of tests to see what kind of adenovirus was present, adenovirus 41 appeared every time.

Dr. Jay Butler is the deputy director for infectious diseases at the CDC. He told the Associated Press that the fact that adenovirus keeps appearing strengthens the possibility that it plays a part in the illness. But it is still unclear how.

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

Dr. Umesh Parashar is head of the CDC group that studies gut diseases caused by viruses. He said recent genetic studies do not suggest that a single new version of adenovirus is to blame.

I'm Jonathan Evans.

Mike Stobbe reported on this story for the Associated Press. Jonathan Evans adapted this story for Learning English.

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inflammation – n. a bodily response to injury or disease in which heat, redness, and swelling are present

nausea – n. a disturbed and unpleasant condition of the stomach; the feeling of being about to vomit

pink eye – n. a contagious infection that causes the eye and inner part of the eyelid to become red and sore