In particular, IFNγ—a cytokine that is produced during inflammation to counteract viral infections and to exert an immunomodulatory influence—and other inflammatory mediators, such as Il-1β, IL-6 and TNFα [51,52], are produced and upregulate indoleamine 2,3-dioxygenase (IDO-1), the rate-limiting enzyme in the catabolism of Trp. Here, IFNG is linked to viral infectious disease.