Health officials are making a renewed call for vigilance against bird flu, but some experts are puzzling over why reports of new human cases have stopped.

Has the search for cases been weakened by government cuts? Are immigrant farm workers, who have accounted for many of the U.S. cases, more afraid to come forward for testing amid the Trump administration’s deportation push? Is it just a natural ebb in infections?

“We just don’t know why there haven’t been cases,” said Jennifer Nuzzo, director of the Pandemic Center at Brown University. “I think we should assume there are infections that are occurring in farmworkers that just aren’t being detected.”