The occurrence of human infections with a novel avian H7N9 influenza virus since 2013 in China demonstrates the continuing threat posed by zoonotic pathogens.Although the first outbreak wave centered on eastern China was seemingly averted,human infections recurred in October 2013-2015.It is unclear how the H7N9 virus re-emerged and how it will further develop,in turn potentially causing a long-term threat to public health.