Nearly 200 cases of severe, unexplained hepatitis have been reported in children worldwide over the last few months, including at least nine cases in the U.S., according to news reports . Here's what we know about the mysterious outbreak: What is hepatitis? Hepatitis is inflammation of the liver. It can be caused by viruses, drugs such as alcohol, prescription medications, toxins or certain medical conditions, according to the U.S. Centers for Disease Control and Prevention (CDC) . Most often, hepatitis is caused by a viral infection — specifically, the hepatitis viruses A, B, C, D and E. In the U.S., hepatitis A, B and C are the most common causes of viral hepatitis, according to the CDC. Hepatitis can cause swelling and damage to the liver, which can affect how well this vital organ functions, according to the National Institutes of Health . Why are the cases unusual? Cases of severe hepatitis, like the ones in the current outbreak, are unusual in children. In addition...