The role of IDO during primary host responses to influenza infection has not been defined, but elevated IDO activity due to prior influenza infection interfered with host control of subsequent Streptococcus pneumoniae lung infections as treatment with the reversible IDO inhibitor 1-methyl-tryptophan (1MT) led to significant reduction in bacterial outgrowth in lungs and to reduced levels of IL-10 and TNFα [6]. Here, IDO1 is linked to streptococcal pneumonia.