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.