IDO1 and viral infectious disease: However, it has been found that various bacterial, fungal, and viral infections can induce the expression of IDO,44, 45, 46, 47, 48, 49, 50, 51, 52, 53 and tryptophan can serve as a prognostic biomarker for COVID‐19, but future research still needs more detailed classification of the pathogen.6, 54