The 2014-15 outbreak is considered the most destructive in the nation’s history. It sent poultry and egg prices soaring and cost the industry more than $3 billion — though the federal government compensated farmers for lost flocks. In the end, nearly 50 million birds were killed by the virus or destroyed to prevent its spread, a vast majority of them in Iowa and Minnesota.

John Burkel, 54, a fourth-generation turkey grower in northern Minnesota, has been watching the spread with trepidation. In 2015, the virus tore through his farm in a matter of days, leaving just 70 survivors in a shed that had held 7,000 birds. The weeks that followed were spent culling, composting the dead and then repeatedly disinfecting the barns.

As a precaution, health officials also advised that he and his son take a course of the antiviral drug Tamiflu. “We’ve never seen a virus that virulent,” said Mr. Burkel, a state legislator who works the farm with his wife and two children. “It was just horrible.”

Since then, agriculture officials across the country have pushed farmers to embrace an array of biosecurity measures aimed at preventing outbreaks. They include sealing up tiny holes that might allow mice or sparrows to enter barns, disinfecting the tires of feed-delivery trucks before they enter a farm and creating “clean” and “dirty” zones where workers can change into fresh footwear and coveralls before stepping inside an animal containment shed.

At the same time, experts say that federal officials have strengthened the nationwide system of surveillance that allows researchers to track, in almost real time, an avian flu’s spread within wild bird populations. “I think the crisis of 2015 made us realize it takes a village to prevent an outbreak and has left us much better prepared,” said Dr. Yuko Sato, a poultry veterinarian at Iowa State University who advises local farmers about improving their biosecurity practices.

But hypervigilance has its limits, especially against a microscopic pathogen that can infiltrate a barn on the leg of a single housefly. For a growing number of scientists, the real threat is the nation’s industrialized system of meat and dairy production, with its reliance on genetically identical creatures packed by the thousands inside huge confinement sheds.

Nearly all the nine billion chickens raised and slaughtered in the United States each year can trace their lineage to a handful of breeds that have been manipulated to favor fast growth and plump breasts. The birds are also exceptionally vulnerable to outbreaks of disease. “They all have the same immune system, or lack of an immune system, so once a virus gets inside a barn, it’s going to spread like wildfire,” said Dr. Hansen, the public health veterinarian.