Chlamydia Chlamydia is the most common sexually transmitted infection in the United States. It strikes about 4 million people a year in the U.S.. 100,000 women become infertile each year from a pelvic inflammatory disease (PID), which plugs up or scars the filopian tubes, so the egg can’t reach the uterus. 45% of sexually active teenagers are exposed to the disease at least once. Clamydia is caused by a bacterium, named chlamydia trachomatis. The pathogen enters the body through sexual activity, or when some comes in contact with the infected persons bodily fluids, such as blood, semen, or vaginal fluid. Chlamydia is often unnoticed because the symptoms are often inconspicuous o