Bovine viral diarrhea virus (BVDV) is classified in the Pestivirus genus within the Flaviviridae family, which also contains the Flavivirus and Hepacivirus genera. Infection of cattle with BVDV induces three disease conditions that range from clinically inapparent to clinically severe. The conditions are termed as congenital persistent infection, mucosal disease, and acute bovine viral diarrhea (BVD). Mucosal disease is a sequel of congenital persistent infection and is caused by a “mixed” infection of noncytopathic BVDV and cytopathic BVDV. Acute BVD occurs after birth and is induced by primary postnatal infection with either cytopathic or noncytopathic BVDV. Acute BVDV may result in respiratory, enteric, and/or reproductive disease. The severity of disease varies from clinically inapparent to fatal and depends on the virulence of the viral strain, physical and environmental stressors, and intercurrent infection with other pathogens. During acute or persistent infection with BVDV, viral replication occurs in a variety of cell types located in the integument, alimentary canal, nervous system, respiratory tract, and immune system. Although many cell types are permissive for viral replication, BVDV has a predilection for cells of the immune system. Viral replication in lymphoid cells may directly, or indirectly, alter immune function and enhance severity of disease during mixed infections of BVDV or mixed infections of BVDV with other pathogens. This chapter provides examples of mixed infections involving BVDV, and discusses the role BVDV may have in enhancing disease processes during a mixed infection.