If the world finds itself amid a flu pandemic in a few months, it won’t be a big surprise. Birds have been spreading a new clade of the H5N1 avian influenza virus, 2.3.4.4b, around the world since 2021. That virus spilled over to cattle in Texas about a year ago and spread to hundreds of farms across the United States since. There have been dozens of human infections in North America. And in some of those cases the virus has shown exactly the kinds of mutations known to make it better suited to infect human cells and replicate in them.

No clear human-to-human transmission of H5N1 has been documented yet, but “this feels the closest to an H5 pandemic that I’ve seen,” says Louise Moncla, a virologist at the University of Pennsylvania. “If H5 is ever going to be a pandemic, it’s going to be now,” adds Seema Lakdawala, a flu researcher at Emory University.