M. avium infection-induced thymic atrophy arises from a synergy between glucocorticoids (GC) and nitric oxide (NO) produced by interferon gamma (IFNγ) activated macrophages (Mφ), since mice lacking IFNγ, iNOS or the activation of Mφ by IFNγ show no infection-induced thymic atrophy (10). This evidence concerns the gene IFNG and infection.