An nationwide influenza epidemic is spreading, doubling the number of cases per hospital, according to data compiled through Tuesday by the National Institute of Infectious Diseases.

 

Over 90 percent of the cases this flu season are due to the Hong Kong influenza A virus subtype H3N2. The data marked the first epidemic in five years of the seasonal flu virus, which can cause encephalitis in infants and gravely affect the elderly, the institute said.

 

Data collected on a regular basis from around 5,000 medical institutions across the country show that the average number of cases stood at 7.33 in the week through Jan. 15, up from 3.76 a week earlier.