The H5N1 strain of avian flu responsible for the deaths of tens of millions of birds in the U.S. in recent months, and countless more worldwide, poses an existential threat to the poultry industry in the U.S.—and a potentially widespread human health threat, experts say.

The strain—first identified in domestic waterfowl in China in 1996—is behind nearly 58 million U.S. bird deaths in the past year. These deaths have occurred both directly due to the virus and indirectly, when flocks are culled to curb further exposure, according to the U.S. Centers for Disease Control and Prevention. The virus has a near 100% fatality rate among birds, killing most infected within 48 hours.

This season’s bird flu outbreak is the worst in U.S. history, having surpassed a 2015 outbreak the CDC once called “arguably the most significant animal health event in U.S. history.” That year, nearly 51 million birds died nationally due to H5N1 and related avian flu viruses. This season’s outbreak is also the worst in U.K. history, with farmers in England ordered to keep their birds indoors as of Nov. 7 in a poultry “lock down” of sorts.