Recent studies on bovine virus diarrhoea (BVD virus) afforded a deeper understanding of the epizootiology of this virus. It is of vital importance to determine whether BVD virus infection occurs within the uterus prior to the 120th day of gestation, during a later stage of pregnancy or after birth. When infection occurs before the 120th day of gestation, normal or abnormal calves which persistently carry the BVD virus, will be delivered. When infection occurs during a later stage of gestation, the calves which are born, will be immune to BVD virus. Symptoms of the disease (directly or indirectly associated with BVD virus) may appear in infections at an age under six months. As a rule, effects of BVD virus infection are slight in older animals. The very severe clinical picture of Mucosal Disease is confined to persistently infected carriers of BVD virus. The most important source of BVD virus infection consists in persistently infected carriers of BVD virus, which continue to excrete large quantities of BVD virus. The diagnosis of BVD virus infection is usually established at a late stage during the course of the disease so that vaccination is no longer relevant. Moreover, it is not clear to which extent immunity of gestating animals, induced by BVD virus vaccines, will be able subsequently to protect a foetus from infection due to BVD virus.