How diseases cross borders can be a dark mystery. It’s never been clear, for instance, just how West Nile virus arrived in the United States in 1999. Researchers think they know why clusters of malaria cases occur around international airports, but no one has proved the hypothesis by catching an infected mosquito as it flies out of a plane’s cargo compartment.

So when piglets started dying in droves in the summer of 2013, from a rapidly spreading disease called porcine epidemic diarrhea virus (PEDv) that had never been seen in the United States before, animal health experts were baffled. In a frantic search for the source, epidemiologists investigated whether the pathogen had escaped from a laboratory, or been carried by migrating birds, or lurked in frozen