Infection with Y induced 2- to 13-fold increases in expression of cytokines and T cell markers tested in brain and thymus (except CD4 mRNA in thymus) at 5 and 7 days pi (3 and 5 days pi for IFN-γ response in brain; 7 days pi for CD8 expression in brain) (P<0.05), relative to infection with PS. This evidence concerns the gene CD4 and infection.