Nearly two-thirds of sexually active young women don't get regular chlamydia transmission, a CDC study finds.

That means more than 9 million young American women don't know whether they've been infected, study leader Karen Hoover, MD, MPH, said in a teleconference from this week's National STD Prevention Conference in Minneapolis.

And the odds of being infected are pretty high: Chlamydia is the most common STD, as well as the most common reportable infection in the U.S.

"There were 1.3 million reported cases of chlamydia in 2010, but the CDC believes the actual number is more than twice that -- 2.8 million new cases each year in the U.S.," Gail Bolan, MD, director of STD prevention at the CDC, said at the teleconference.

Among women, nearly 5% of 19-year-olds and more than 1% of 15-year-olds are infected. Men are at least as likely to be infected. But it's women who suffer the most severe consequences. That's because chlamydia infection often is silent -- without symptoms -- until the infection becomes more serious.

Left untreated, 10% to 15% of women will get pelvic inflammatory disease (PID). And up to 15% of those women will be left infertile. And some will die from chlamydia-related ectopic pregnancy.

The CDC recommends an annual chlamydia test for any sexually active woman age 25 and younger. Women over age 25 should get annual tests if they have a new sex partner or have multiple sex partners. Routine screening isn't recommended for men.

When diagnosed, chlamydia is easily treated. But treatment isn't permanent, as Kelly Morrison Opdyke, MPH, and colleagues found in another conference presentation.

Opdyke's Cicatelli Associates Inc. team studied 63,774 people who tested positive for chlamydia from 2007 to 2009. They found that 25% of men and 16% of women have a new chlamydia infection when retested within six months.

And those are just the people who get another test. People who show up for screening tests tend to be healthier than those who don't. Yet only 11% of men and 21% of women got that chlamydia retest in the Opdyke study.

Those who test positive for chlamydia are supposed to get a repeat test three months after treatment. Sex partners should be evaluated and treated as well. Women are at increased risk for reinfection if their sex partners have not been treated appropriately.

Unfortunately, you might not be able to rely on your health care provider to offer you that test.

California Department of Health researcher Holly Howard, MPH, and colleagues studied six of their state's large family planning clinics. They found that only 70% of patients were retested for chlamydia or for gonorrhea, the second most common STD.

When the clinics installed pop-up reminders on patients' computer records -- using existing billing software -- the retesting rate went up to 86%.

To remind patients to ask for chlamydia retests, SUNY Buffalo researcher Gale Burstein, MD, MPH, and colleagues used a simple email system. Four to five weeks after testing positive for chlamydia or gonorrhea, students got an automated email reminder. That was followed by a personal email and, if needed, a telephone call.

What happened? Retest rates for chlamydia went from 16% to 89%.

"We must not only increase chlamydia screening rates but ensure re-testing," Bolan said. "And we must encourage and support individuals' efforts to protect themselves. This may mean abstaining from sex, reducing the number of sex partners, or proper condom use."