In contrast to the CD8 T cell compartment of SPF laboratory mice, which consists primarily of naïve T cells and is similar to that of a neonatal human, sequential infections with common pathogens or co-housing laboratory mice with wild/pet store (“dirty”) mice generates a CD8 T cell compartment that is similar to adult humans and is comprised of a large number of Ag-experienced CD8 T cells with increased representation of cells in peripheral tissues (103). This evidence concerns the gene CD8A and infection.