ISG15 and viral infectious disease: As in mammals, type-I IFN are responsible for the induction of a high number of genes (Interferon Stimulated genes, ISG) after virus infection in fish and especially in rainbow trout, including conserved antiviral genes such as rsad2/viperin [14], mxa [15], and isg15 [16, 17], which constitute the first cellular line of defense against viral infection [18–21].