ABSTRACT Chlamydia trachomatis (CT) is the most common bacterial sexually transmitted infection (STI) worldwide and causes irreversible reproductive tract damage in women. As most CT-infected women have no symptoms and are unaware that they are infected, the CDC recommends annual CT screening in women aged <25 and high risk women to detect and treat asymptomatic infections and prevent complications; in men, routine screening is not recommended. Despite 3 decades of screening programs, CT rates in the U.S. have dramatically increased, driven primarily by increasing CT reinfections. As most CT-infected women are heterosexual and routine screening in men is not recommended, undetected CT infections in men are the reservoir that propels reinfections in women. To curb the CT epidemic, it is critical that we understand whether men can develop immunity to CT and identify factors that promote CT reinfections (e.g., asymptomatic men who shed significant CT organisms). Approximately 1/3 of men who present for treatment as a CT contact (i.e., their sex partner was confirmed to be CT-infected) are uninfected, suggesting that these men either cleared CT infection by immune- mediated mechanisms or have a urethral microbiome that resists CT acquisition. To prevent CT reinfections in women, this proposal focuses on male contacts to CT-infected women and will study if CT infection is associated with specific microbiome compositions, comorbid STIs, or adaptive T-cell and antibody immunity. To do this, we will enroll up to 400 male CT-contacts, collect blood and urine specimens, treat them with the CDC-recommend first-line antibiotic, and ask them to return for a 1-month test of cure (TOC) visit (Aim 1). To study if CT uninfected contacts to CT have long-lasting immunity and how the urethral microbiome and immune system change over time, we will re-enroll 100 men for a 6-month longitudinal study to collect weekly urine specimens and symptom/sexual behavior diaries and blood and urine specimens at 3-month and 6-month screening visits. Weekly urine specimens will be stored and tested at study completion to create a natural history study. By studying the rates of incident CT infection over time, we will be able to elucidate if resistance to CT infection, defined in Aim 1, correlates with (1) increased rate of natural clearance between visits; (2) decrease in incident CT infection at 3- or 6-months; (3) decrease in incident CT infection organism load; and (4) decrease in the rate of homologous CT genovars. Study findings will inform novel approaches to preventing transmission from men to women and help curb rising CT rates and complications.