IL-18 was initially shown to synergize with IL-12 for IFN-γ production by Th1 cells in vitro (Robinson et al., 1997), but its essential role in promoting Th1 responses to infection was not always confirmed in the context of in vivo infection (Haring and Harty, 2009; Monteforte et al., 2000). This evidence concerns the gene IL18 and infection.