When I was a youngster back on the family farm in Michigan 1 lot of pigs started getting sick. The local vet came out and killed one of the sick runts and opened it up. He looked at the diaphram with a magnifying glass. He showed us lot'sa little white circles and said that was the culprit. Some sort of parasite that lodged there and it would kill them. So we shot the entire pen to keep it from the others. IIRC it was Trichinosis, but that was many mons ago and I'm not 100% sure anymore.