A patient with the H7N9 avian flu is treated in a hospital in Wuhan, in central China’s Hubei province, in February of this year. The 2017 outbreak was the deadliest in China since H7N9 first appeared in humans in 2013.

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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…