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