Transmissible gastroenteritis (TGE) virus, a coronavirus, causes an acute infection of the small intestine of the pig. The disease is rarely fatal in adult animals but can cause extensive mortality in the neonate. Since its first description in Britain in the 1950s, the disease has been epizootic but recently it has become established on some farms and an antigenically related respiratory virus has become endemic in the national herd over the past two years. Piglet immunity to TGE, which relies on passive protection from milk, has been impossible to achieve with vaccines and research has aimed at understanding the nature of the interaction between virus and the pig. Following infection of the pregnant sow, antibody-producing cells migrate to the mammary gland where they secrete virus neutralising antibodies into the milk; prospective vaccines will need to stimulate a similar response. The location and number of antigenic sites on the virus particle associated with neutralisation have been established with monoclonal antibodies and the role of the other viral genes in pathogenesis and immunity is being studied with genetic engineering techniques.