Based on human viral infection studies, a linear model of T cell differentiation has been proposed wherein CD27+CD28+CD45RA+ naïve cells progress through a CD27+CD28+CD45RA- early antigen-experienced phenotype and then proceeds to a CD27+CD28-CD45RA−/+ intermediate phenotype and finally to CD27-CD28-CD45RA+/− late antigen-experienced phenotype. This evidence concerns the gene CD28 and viral infectious disease.