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 semen taken from hogs in other countries. Was pig feed contaminated? Were feral swine carrying it? The virus was closely related to one found in China, and many of the ingredients in pig feed come from there. But the Chinese meat firm Shanghui was in the midst of buying Smithfield Foods; had a businessman unknowingly carried it around the world?

As the epidemic spread, millions of pigs died, more than 10 percent of the entire American herd. The US was virgin territory for the virus, which leapfrogged the country randomly and with wicked speed. It killed so many pigs that pork prices spiked, and it cost more than $1.8 billion to track and detect the disease, and to clean up infected areas and shore up the earnings of affected farms.

Two years later, after exhaustive studies, the US Department of Agriculture’s Animal and Plant Inspection Service says it has identified the vehicle that carried the virus across the world. The culprit was something so humble, it was easy to overlook: the giant woven bags that are used to transport feed ingredients across oceans, and then reused in the United States to carry mixed feed between feed mills and onto farms.

Until now, there have been no regulations governing the use, reuse or resale of the bags, the USDA said—and that includes any rules about cleaning or disinfecting them. So a bag that picked up contamination abroad, probably in China, could be moved in the US from port to wholesaler to feed mill to farms and on to other feed mills, disseminating virus as it went.

In their research, epidemiologists built an algorithm that would help them rule out less plausible possibilities and narrow down the cause. The USDA report says that, out of 17 possible explanations, only the giant bags—which can hold up to 3,000 pounds of material—tick every box. They could have been contaminated outside the US; they would have protected the virus in transit; they were moved around the country in a way that would rapidly disperse the pathogen; and they were carried onto farm properties, breaching biosecurity protections that farmers had set up.

View Images FIBCs holding fertilizer on a French farm. Original: Wikimedia (CC)

The bags, called “Flexible Intermediate Bulk Containers,” or FIBCs (or GRVS, if you conduct business in French), are incredibly common in shipping. They are a huge business, with dozens of manufacturers and their own international trade show, and millions of them are used each year to move things that are heavy and dry: wood shavings, gravel, granular chemical compounds, vitamins, grains and pet food. They have big loop handles, so they are easy to tie closed, or to hoist with a forklift or a hook. They are stackable and stable, because they conform to the shape of what’s inside them; and rip-resistant and lightweight, because they are made of woven synthetic.

And, it turns out, they are accidentally well-constructed for giving a home to viruses. To keep from tearing, FIBCs are assembled with an outer layer and a lining. In tests, the USDA found that once a bag was contaminated—by particles of ingredients sifting through the inside, or tainted water soaking through the shell—the shield of the outer layer could protect the virus from sun, wind and temperatures for weeks.

That could explain something that people noted early on in the epidemic: Outbreaks seemed to have something to do with pig food. When investigators checked, though, they could not find any link among all the farms. There was no single person—no veterinarian or field technician or truck driver—who had visited every affected property. The farms did not feed the same food or use the same supplements or buy from the same companies. What investigators missed, until the report’s deep dive, was that almost every large pig farm in the country uses the same types of feed ingredients—and all the ingredients move along a transport chain that was polluted by the arrival of contaminated bags.

It’s not clear what happens next, though. If bags being transported into the US were contaminated in another country, that is outside the purview of US regulators. Inspections of imported food, even animal food, are required under US law, but the corps of inspectors is known to be understaffed. And import inspection might not even have detected virus lingering in minuscule amounts within the woven surfaces of the bags.