IDO1 and infection: Tryptophan catabolism is very likely due to the induction of IDO by pro‐inflammatory cytokines, in particular IFN‐γ.11, 12, 13 Most data about IDO induction and influenza virus infection are derived from mouse models: Yoshida et al showed a 120‐fold increase in IDO activity in murine lungs after infection with PR8 influenza virus.25 Huang et al investigated IDO1 knockout mice after influenza PR8 infection and showed that genetic IDO ablation led to much faster recovery after virus clearance.26