They tested negative for the viruses that normally cause hepatitis.

Besides viruses, other causes of liver diseases include bacteria, parasites (e.g. schistosomes), genetics, metabolism, and alcoholic liver disease. After investigation, no supporting evidence for any of these causes was found.

Therefore, doctors are still investigating other viruses. Currently, the most suspected virus is adenovirus.

they were likely infected by adenovirus. but the wrench in the works here is that:

that adenovirus generally causes relatively severe diseases only in immunocompromised children. Therefore, adenovirus infection alone is insufficient to fully explain the severity of this outbreak.

Brings to mind that guy who used to say sars = hivs. this is a really startling outbreak.