As many as 1 in 20 teenage girls and women and more than 2 percent of the general population in America are infected with chlamydia, a sexually transmitted disease, a new survey has found.

Pregnant women attending publicly funded clinics and economically disadvantaged youth are especially at risk of the bacterial infection, which can cause serious problems including infertility if untreated, the Centers for Disease Control and Prevention survey found.

“STDs often have no symptoms and therefore frequently go unrecognized and undiagnosed,” said Dr. John Douglas, director of the CDC’s STD prevention programs.

“Stepping up screening and prevention efforts is critical to ensuring that young people do not suffer the long-term effects of untreated chlamydia, including infertility,” Douglas said in a statement.

Federally funded efforts have prevented millions of infections and saved an estimated $5 billion in direct medical costs over the past 30 years, the CDC told a meeting of the International Society for Sexually Transmitted Diseases Research in Amsterdam.

CDC researchers found 2.2 percent of U.S. adults aged 14 to 39 had chlamydia. Nearly 1 in 20 women between the ages of 14 and 19 - 4.6 percent - were infected.

For the study CDC researchers analyzed answers to the National Health and Nutrition Examination Survey, a lengthy survey of tens of thousands of Americans taken between 1999 and 2002.

Economically disadvantaged young adults aged 16 to 24 and enrolled in a national job-training program had a high risk, with one in 10 infected with chlamydia.

And 5.8 percent of 86,000 women aged 15 to 45 attending publicly funded prenatal clinics in 18 states were infected.

In 2003, 877,478 cases of chlamydia were reported in the United States, making it the most commonly reported STD, the CDC said.

Chlamydia is easily cured with antibiotics, but is often undiagnosed because of its causes few symptoms. Besides infertility, the infection can cause pelvic inflammatory disease, ectopic pregnancy, and chronic pelvic pain.