Chlamydia, America's most common sexually transmitted disease, is most often seen in teens and young adults, report CDC researchers.

Teen girls had the highest number of cases. Numbers were also high in among blacks, as well as among teens and young adults from economically deprived backgrounds.

The CDC's figures were presented in Amsterdam at a meeting of the International Society for Sexually Transmitted Diseases.

Read Web MD's "Get the Facts about Chlamydia."

About Chlamydia

Chlamydia is a common sexually transmitted disease caused by a bacterial infection. It is spread by vaginal, oral, or anal intercourse. Chlamydia is known as a "silent" disease because about three-quarters of infected women and about half of infected men have no symptoms, according to the CDC. When symptoms appear they do so weeks after exposure.

It can be cured by antibiotics if the drugs are taken exactly as advised.

If not treated properly, complications can arise from the infection. In women, untreated infection can spread into the uterus or fallopian tubes and cause pelvic inflammatory disease (PID) or permanent damage and infertility. Complications in men are rare, but the infection can spread in men and lead to infertility.

It's estimated that half to three-quarters of babies born to women with chlamydia get the infection. Between 30 percent to 40 percent of babies infected with chlamydia at birth develop complications, such as conjunctivitis or pneumonia.

Having had chlamydia does not protect you from getting it again.

Risk factors include having sex without condoms, having more than one sex partner, having a high-risk partner or partners, and starting sexual activity before age 18.

Read Web MD's "Only 1 in 4 Young Women Get Chlamydia Tests."

STDs May Not Be Obvious

It's possible to have chlamydia and not know it.

Sexually transmitted diseases [STDs] "often have no symptoms and therefore frequently go unrecognized and undiagnosed," says the CDC's John Douglas Jr., MD, in a news release.

Douglasdirects the CDC's division of STD prevention.

Read Web MD's "How Can I Prevent Chlamydia?"

National Picture

Some of the CDC's numbers came from a national health survey. More than 6,600 people aged 14-39 were tested for chlamydia and another STD, gonorrhea.

Among the findings:

—Chlamydia was present in 2.2 percent of the group.

—Men and women had a similar number of chlamydia cases.

—For women, chlamydia was most common in the late teens.

—Nearly 1 in 20 women aged 14-19 tested positive for chlamydia.

—Among men, chlamydia was most common from 20-29.

—3.2 percent men in their 20s had chlamydia.

—Chlamydia was seen in about 6.4 percent of blacks, compared with 1.5 percent of whites.

Read Web MD's "Your Guide to Sexually Transmitted Diseases."

Economic Hardship a Risk?

Another study focused on more than 150,000 people aged 16-24. All were socioeconomically disadvantaged. Roughly one in 10 were infected, writes the CDC.

They were screened for chlamydia when they signed up for a national job training program.

The results showed chlamydia in:

—11 percent of the more than 106,000 women

—8 percent of the nearly 50,600 men

—About 13 percent of 16-year-old women

—Nearly 9 percent of men aged 20-24

—8 percent of men aged 16-19

A third study checked for chlamydia in women aged 15-24 at prenatal clinics.

Out of more than 86,000 tests, almost 6 percent were positive for chlamydia. Again, the disease was most common in younger women and blacks.

Visit WebMD's Sexual Conditions Health Center

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: International Society for Sexually Transmitted Disease Research, Amsterdam, July 10-13, 2005. News release, CDC.