This past year China had the largest outbreak of a deadly bird flu since the virus was first detected in March 2013.
For the past five years, China has had annual waves of H7N9 outbreaks that peak around January and February.
During the 2017 season, the country reported nearly the same number of cases as all four previous years combined, researchers at the U.S. Centers for Disease Control and Prevention report Thursday. The virus cropped up in more geographic regions. And it showed signs of evolving in ways that cause concern.
As NPR reported in April, the virus has picked up mutations that make it more deadly in poultry and less susceptible to antiviral treatments. "Our research shows it can kill all the chickens in our lab within 24 hours," virologist Guan Yi told NPR.
H7N9 isn't your run-of-the-mill bird flu. H7N9 is "the influenza virus with the highest potential pandemic risk," the CDC writes in the journal Morbidity and Mortality Weekly Report.