Cattle of all ages are susceptible to infection with bovine viral diarrhoea virus (BVDV). BVDV is a single linear positive-stranded RNA virus in the genus Pestivirus of the family Flaviviridae. BVDV infection results in a wide variety of clinical manifestations, including enteric and respiratory disease in any class of cattle or reproductive and fetal disease following infection of a susceptible breeding female. BVDV viruses also cause immune suppression. Animals that survive in-utero infection in the first trimester of gestation are almost always persistently infected (PI). PI animals provide the main reservoir of the virus and excrete large amounts of virus in urine, faeces, discharges, milk and semen. The virus spreads mainly by close contact between PI animals and other cattle. This virus may also persist in the environment for short periods or be transmitted with contaminated reproductive materials. Vertical transmission plays an important role in its epidemiology and pathogenesis. Infections of the breeding female may result in conception failure or embryonic and fetal infection, which results in abortions, stillbirths, teratogenic abnormalities or the birth of PI calves. While BVDV strains are predominantly pathogens of cattle, interspecies transmission can occur following close contact with sheep, goats or pigs. There is no standard vaccine for BVD, but a number of commercial preparations are available. BVDV is not considered to be a human health hazard.