In order to develop an overview of the adaptive response to Chlamydia infection, we initially examined the kinetics of bacterial growth and Chlamydia-specific CD4 T cell expansion during systemic infection with Chlamydia. When C57BL/6 female mice were infected intravenously (i.v.)with 1×105 inclusion-forming units (IFUs) of C. muridarum, the bacterial burden in the spleen peaked around day 4 post-infection and decreased quickly thereafter (Fig. 1A). The gene discussed is CD4; the disease is infection.