The record-breaking nationwide outbreak of gastroenteritis caused by norovirus may stem from a mutation of the disease, an expert said Wednesday.
The highly contagious virus has typically been transmitted by eating raw oysters and clams, but this season's epidemic comes mostly from human-to-human infections and can best be explained by a possible outbreak of a new virus strain by, for example, mutation, said Shigeo Matsuno, a senior researcher at the National Institute of Infectious Diseases.
"The cause . . . is not eating raw oysters," Matsuno said. "It must be human-to-human infection in most cases. It is natural to think that the infection spread so rapidly because many people have no immunity to the virus."