Chlamydia trachomatis is the single most important infectious agent causing blindness worldwide and also causes sexually transmitted infections in the US. There are currently no vaccines available to prevent these diseases. The CDC estimates that 4.2% of all young adults are now infected with Chlamydia, making this one of the most prevalent bacterial infections in the US. Unfortunately, young women with untreated Chlamydia infection can develop serious pelvic inflammatory disease (PID), infertility, chronic pelvic pain, or suffer ectopic pregnancy. The current consensus among scientists and clinicians is that an effective Chlamydia vaccine is needed and greater understanding of immunity to Chlamydia infection is required to achieve this goal. Work in our laboratory attempts to directly examine the protective role of CD4 T cells in a mouse model of genital infection in order to understand the requirements for developing a vaccine.