BVD virus has an immunosuppressive effect and thereby favours infections with secondary or opportunistic pathogens. Of special importance is the synergy between BVDV and Pasteurella haemolytica type 1, which is characterized by a severe antibiotic-resistant pneumonia [80]. BVDV is thought to have an enhancing effect in the case of various other diseases: actinomycosis, stomatitis papulosa, salmonella enteritis, E. coli, acute helminthiasis, mastitis, metritis. In many cases a participation of BVDV appears covert in view of a lack of symptoms typical of BVD. While immunotolerance is highly specific, immune suppression is unspecific. Important mechanism of immune suppression include an in vivo induction of apoptosis of B and T cells in lymphoid tissue, a downregulation of MHC II and IL-2 or an activation of a T-helper 2 response by ncp BVD [81].