Porcine reproductive and respiratory syndrome (PRRS), also known as ‘mystery disease of swine’ and ‘blue-eared pig disease’, has become one of the most important diseases of pigs in spite of the fact that it is no longer listed by the World Organisation for Animal Health (OIE). It made its debut in North America in about 1987 and in Europe (Germany) in 1990, but serological evidence suggested that it was present for some time in pig populations before clinical signs were observed, including in South Korea in imported pigs in 1985, and outbreaks occurred in Japan in 1988 and Taiwan in 1991. It manifested in commercial piggeries as a severe disease characterised by high fever, abortions in sows, high mortality in piglets and pneumonia in weaners and growers. The agent proved difficult to pin down but was isolated almost simultaneously in The Netherlands and USA and proved to be a previously unknown virus in the genus Arterivirus of the family Arteriviridae. Its importance lies in the negative effects that it has on high-producing herds in some of the top pig-producing countries of the world and the difficulty of eradicating it once it has become established.