We know a lot about bovine viral diarrhea virus (BVDV) in cattle, but we still have a lot to learn about persistent BVDV infection. Cattle persistentlyinfected (PI) with BVDV are lifelong carriers and shedders of BVDV. A PI animal is defined as one that was infected at 40-150 days of gestation. The developing immune system of the fetus actually recognizes the virus as part of itself, which is referred to as immunotolerance.