IDO1 and viral infectious disease: A simple interpretation of this positive correlation is that the production of IFN-γ, a cytokine which is known to induce the expression of both CXCL10 and the metabolic enzyme IDO31, by the immune system correlates with the level of viral infection early in the disease, acting on immune cells or lung parenchymal cells to induce CXCL10 and IDO, possibly along with other metabolic enzymes.