Porcine epidemic diarrhea virus has killed hundreds of thousands of piglets since it was first identified on an Ohio farm earlier this year, according to estimates, and veterinarians and producers are scrambling to stem its spread. "Every pig in the United States is susceptible," said veterinary professor Robert Morrison of the University of Minnesota, noting that the U.S. swine population carries no immunity to the disease. Testing to identify the source of the infection in the U.S. has been unsuccessful, but experts continue to search. "If PEDV can get into the United States, what about some of the even more nasty viruses?" said veterinarian Tom Burkgren, executive director of the American Association of Swine Veterinarians.