The infection of intestinal commensal bacteria up-regulates the expression of IL-25 by intestinal epithelial cells and limits the expansion of Th17 cells in the intestine via inhibiting the expression of macrophage-derived IL-23, indicating that commensal bacteria influence intestinal immune homeostasis via direct regulation of the IL-25-IL-23-IL-17 axis (119). The gene discussed is IL25; the disease is infection.