IDO1 and viral infectious disease: Previous work has shown that tryptophan metabolism by indoleamine-2,3-dioxygenase (IDO) (17, –, 20) is upregulated during viral infections (18, 38), and significant changes to the kynurenine-tryptophan ratio (KTR) could be due to increased monocyte activation and are associated with microbial dysbiosis (27).