“During 2007, approximately 1.1 million cases of chlamydia were reported to CDC; more than half of these were in females aged 15 to 25 years.”
It often causes few or no symptoms and if not treated with antibiotics can cause pelvic inflammatory disease, infertility, chronic pain and ectopic pregnancy, which is a pregnancy outside the uterus — something that can kill the mother or the baby.
Men and women alike can be infected with chlamydia and can re-infect one another if only one sexual partner is treated. It can make men sterile, too, but only rarely.
Chlamydia can be passed to a newborn and can cause pneumonia and conjunctivitis, or pink eye.
The CDC recommends yearly chlamydia testing of all sexually active women 25 or younger, older women who have a new sex partner or multiple sex partners, and all pregnant women.