CD4 and disease arising from reactivation of latent virus: Our data showed a significant decrease in viral growth rate and almost undetectable expression of lytic genes in isolated CD4+ T cells at 10 dpi (Fig. 4), suggesting that MDV established a latent infection, whereas the significant increase in viral growth rate and expression of lytic genes at 14 dpi (Fig. 4) indicates that MDV reactivation was induced, which is consistent with the previously reported replication cycle of MDV in chickens (37).