ATLANTA (CNN) -- Almost one in 10 female Army recruits have chlamydia infections, and the rate is highest among those who are black or still in their teens.

Chlamydia, a sexually transmitted disease, has long been recognized as a common infection, especially among young women, but the new study offers one of the most comprehensive looks at its prevalence across the country.

Researchers from Johns Hopkins University tested 13,204 new female recruits during basic training at Fort Jackson, S.C., in 1996 and 1997. The average age was 21. Half of the women were white, one-third were black and the rest were of other races.

"These findings have national applicability," lead researcher Dr. Charlotte Gaydos of Johns Hopkins Medical Institution said in an interview. "We had a (test subject) population from all 50 states and four U.S. territories."

Gaydos' team tested urine samples to see which ones were most susceptible to chlamydia, which can inflame the pelvis, lead to tubal pregnancy and cause infertility in women.

The study found that 9.2 percent had chlamydia, but the risk fell as their age increased. The highest rate was among 17-year-olds: 12 percent of them were infected.

Race and geography also played a major role. Black women were three times as likely as whites to have chlamydia, and the highest rates were seen among women from the South. More than 15 percent of the recruits from South Carolina, Georgia, Alabama, Louisiana and Mississippi were infected.

The study, directed by Dr. Charlotte A. Gaydos, was published in Thursday's issue of the New England Journal of Medicine.

More than 4 million chlamydia infections are estimated to occur in the United States each year. The infection is especially likely to go untreated in women, because they often have no symptoms.