BOVINE viral diarrhoea virus (BVDV) is an outstandingly successful virus. It is best known as the cause of a variety of clinical conditions resulting in economic losses in cattle, but understanding its biology has shown that BVDV and other pestiviruses have features unique among viruses. View this table: Veterinary Record publication times BVDV causes both transient and persistent infections and can escape the host's immune responses during both events. Transient infection occurs in cattle of all ages. Oronasal infection results in transient viraemia and virus excretion is low before it is eliminated by a standard immune response. The innate interferon response is followed by adaptive cell-mediated and humoral responses so that specific anti-BVDV antibody can be detected within three weeks of infection. Antibody levels continue to rise over the next two months and a solid immunity to that virus is maintained for years. However, if infection occurs in a pregnant animal, the virus escapes by crossing the placenta to the fetus where it infects a wide variety of cells without killing them. The tight bovine placentation prevents the dam's immune response from assisting the fetus. Before 120 days' gestation, a fetus lacks a mature adaptive immune response; all viral antigens are accepted as ‘self’ and are forever seemingly ignored by the fetus' or calf's cell-mediated and humoral adaptive responses. The fetus is immunotolerant to those viral antigens and will not respond to them throughout the rest of its life. The virus has simply escaped the host's adaptive immune response resulting in an animal that is persistently infected. The virus also has to circumvent the host's innate immune response, which is fully functional from the embryo's early development, from the first day of fetal infection throughout the host's life. BVDV is able …