In the latent phase, infected chickens show no clinical signs, but cellular immunity is continually inhibited by apoptosis of CD4+ T cells, CD8-down regulation in CD8+ T cells, decrease in the responsiveness to the stimulation through T cell receptor (TCR) in CD4+ and CD8+ T cells and MHC class I-down regulation at 2–3 weeks after infection [3,4]. This evidence concerns the gene CD4 and infection.