More than half of all chlamydia cases go undiagnosed, according to a new analysis by the Centers for Disease Control.

Just over a third of sexually active women between the ages of 15 and 25, the age group most at risk, get tested annually as the CDC recommends.

Chlamydia is the most common sexually transmitted disease, with 1.3 million diagnosed cased in 2010. The CDC estimates the actual number of new infections is closer to 2.8 million annually.

"Testing rates are far too low across the board," says Karen Hoover, a medical epidemiologist with the CDC. "First and foremost, we have to do more to ensure health care providers know sexually active women should be screened every year."

The good news, Hoover says, is that nearly half of women with multiple sexual partners, who are most likely to be exposed to the disease, get tested annually. The disease can be easily treated with antibiotics, she says.

Among women who were treated for chlamydia, less than a quarter were retested within six months as the CDC recommends. About 16 percent of women are expected to be reinfected in that time frame.

"Re-infection is common because of untreated infection in their partners," says Kelly Opdyke, who studied retesting rates for the CDC "A substantial number of reinfections may be missed."

Women infected with chlamydia often live symptom-free for several years, but the disease can cause potentially fatal ectopic pregnancies, infertility, and pelvic pain in the long-term. In men, the disease often causes painful urination.