British researchers have reported a breakthrough in the study of mysterious cases of hepatitis affecting young children, finding that the serious liver disease is associated with co-infection with two common viruses but not with a coronavirus, Medical Xpress writes.  The World Health Organization (WHO) has reported at least 1,010 probable cases of the disease, including 46 cases that required transplants, and 22 deaths from the disease since last October. Previous theories had suggested that a spike in common adenovirus infections was behind the cases. But in two new studies conducted independently and simultaneously in Scotland and London, scientists found that another virus, AAV2 (adeno-associated virus 2), which was present in 96 percent of all patients examined, played an important role. Normally, AAV2 does not cause disease and cannot replicate without the presence of another "helper virus." Both groups concluded that co-infection: either AAV2 and adenovirus or sometimes herpes virus HHV6 best explained severe liver disease.  Both groups of scientists ruled out recent or prior SARS-CoV-2 infection as a proximate cause. No coronavirus was found in the patients' livers, and although the Scottish study found that two-thirds of patients had antibodies against coronavirus, the rate was similar to the overall prevalence among children at the time. It is not yet clear why hepatitis cases have increased recently, but both groups emphasized the possibility that enclosed spaces may have contributed, either by reducing children's immunity or by changing the way the virus circulates.