Over the past few weeks, countries around the world have reported an unexpected increase in the number of children with hepatitis. So far about 200 cases have been reported. More than half have come from the UK, but there have also been reports from Spain, Japan and the US, among others. Although this is still a very rare disease, it is severe, with 10% of affected children needing a liver transplant. So what might explain this unusual rise? Guardian science editor Ian Sample speaks to Prof Deirdre Kelly about the current theories as to what could be happening, and how concerned we should be. Help support our independent journalism at theguardian.com/sciencepod