Bovine viral diarrhoea has for decades regularly been in the headlines as an economically significant disease of cattle. It is aviral disease (i.e. a disease caused by a virus) that appears in different forms. A majority of BVD infections are without symptoms, i.e. their occurrence is not noticed by the farmer, some animals, however suffer from severe diarrhoea, fever, nasal and ocular discharge as well as  erosions at the muzzle. Naturally, milder forms of the disease can also occur. Mucosal disease, which is generally dreaded, is the most severe form and is always lethal. It occurs exclusively in so-called persistently infected animals, i.e. in animals that harbour and shed the virus throughout their lives.