Recent evidence suggests the existence of the configurations necessary for Th1 cell differentiation in various bony fish species, including the cytokines IL-2, IL-27, and IL-12 [65–67] and transcription factors T-bet, STAT1, and STAT4 [68–70], and it is reported that their expression is markedly induced during bacterial or viral infection. This evidence concerns the gene IL2 and viral infectious disease.