In the spring of 2013, a killer showed up on farms in the Midwest. By the end of the summer, hundreds of thousands of piglets lay dead in its wake.

The menace terrorizing hog farmers is the porcine epidemic diarrhea virus, or PEDV. No one knows where it came from or how it entered the United States, but all pigs are at risk of being infected.

For some reason, PEDV kills only piglets less than one week old. Severe diarrhea and vomiting are followed by death in virtually all cases. Older swine usually recover, and PEDV is not a risk to food safety or to people.

Once PEDV appears on a farm, it’s almost impossible to stop. So until a vaccine is developed, the focus has turned to enforcing quarantine protocols. Considering that an average pig farm hit with PEDV could lose 5,000 piglets and up to $200,000, the stakes are high.