PI calves are created when a pregnant cow at 20-150 days of gestation is exposed to BVD.  In a BVD infected herd, young calves are the most likely animals to be PI.  Herds with a calving season longer than 90 days have cows calving after the breeding season has already started.  This makes it difficult to test all of the newborn calves and remove PIs before they have exposed pregnant cows.