noncytopathic strains of BVDV are endemic in cattle populations. If a fetus is
infected in-utero, it becomes tolerant to the noncytopathic strain and persistently infected. If this animal is
later super-infected with a cytopathic strain or the noncytopathic strain mutates to a cytopathic strain, the
animal is incapable of mounting an appropriate immune response because of tolerance and the virus
causes severe disease