There are some 7,600 pig farms in Pennsylvania, containing from a few to 12,000 pigs. Farmers had to start over with clean pigs, usually at their own cost. The answer is still unclear.
Some researchers believe it can travel long distances through the air.
It may survive temporarily in soil or water. Although most wild animals are killed by it outright, there's a suspicion rodents can harbor the virus and spread it to pigs. Flies may also be a vector. It's more a nuisance than a health threat. It cannot be passed to humans.