Less than half of American women are being tested for chlamydia, according to the Centers for Disease Control and Prevention (CDC). While screening rates have gone up from 25 percent in 2000 to about 42 percent in 2007, not enough women are being tested, the CDC says.

“Chlamydia trachomatis infection is the most common bacterial sexually transmitted disease (STD) in the United States, with more than 2.8 million new cases estimated to occur each year,” researchers wrote in the CDC’s Morbidity and Mortality Weekly Report.   

During 2007, health agencies reported about 1.1 million cases of chlamydia to the CDC; more than half were among women ages 15 to 25 years old. The CDC recommends yearly tests for all sexually active women 25 or younger as well as all older women who have a new sex partner or multiple sex partners and all pregnant women.

Like many other sexually transmitted infections (STIs), chlamydia in many people is asymptomatic (showing no visible signs of having the disease). Untreated, chlamydia can cause pelvic inflammatory disease, infertility and chronic pain.

Read more about chlamydia and other sexually transmitted infections here.