Persistently infected calves are created when a pregnant cow at 20-150 days of gestation is exposed to BVD. One of the best ways to prevent the creation of PI calves is to prevent exposure of pregnant cows to other cattle that could be shedding BVD. This exposure may occur through BVD infected animals in the same pasture, or by exposure to animals in an adjacent pasture. The BVD virus is commonly transmitted by direct contact, but it can also be spread short distances through the air (via nasal and salivary secretions).