Virulent C. psittaci exacerbates host immunosuppression and enhances secondary avian influenza virus serotype H9N2 infection by impairing macrophage functions and upregulating T helper 2 (Th2) cytokines, as well as the expression of interleukin 4 (IL-4) and IL-10 (2, 3). This evidence concerns the gene IL10 and infection.