It was first isolated in mortalities occurring in natural populations of white perch ("Morone americanus") and striped bass ("M. saxatilis") in 1963 in Chesapeake Bay, USA ("Snieszko "et al.", 1964"). Since 1969, this disease has been one of the most important in Japan, affecting mainly yellowtail ("Seriola quinqueradiata") (Kusuda & Yamaoka, 1972). From 1990 it has caused economic losses in different European countries including France (Baudin-Laurencin "et al.", 1991), Italy (Ceschia "et al.", 1991), Spain (Toranzo "et al.", 1991), Greece (Bakopoulos "et al.", 1995), Turkey (Canand "et al.", 1996), Portugal (Baptista "et al.", 1996) and Malta (Bakopoulos "et al.", 1997). Gilthead sea bream ("Sparus aurata"), seabass ("Dicentrarchus labrax") and sole ("Solea" spp.) are the most affected species in Europe Mediterranean countries, as well as hybrid striped bass ("M. saxatilis" x "M. chrysops") in the USA. However, the natural hosts of the pathogen are a wide variety of marine fish (Romalde & Magariños, 1997).