Another infection rages silently in young adults.

Two new studies highlight the importance of another sexually transmitted disease. Caused by the bacterium Chlamydia trachomatis, this infection is common among teens and young adults.

Public health officials say there are more than 4 million new cases of chlamydia diagnosed each year--making it the most common infectious disease in the United States. Most infected people experience no symptoms. If left untreated, however, the disease can cause serious problems. Women can suffer from pelvic infections that lead to infertility.

In the past, a pelvic exam was required to diagnose the disease. However, in the mid-1990s, a highly accurate urine test for the infection became readily available. Thomas C. Quinn of the National Institute for Allergy and Infectious Diseases in Bethesda, Md., and the Johns Hopkins University in Baltimore and his colleagues used the test to study the prevalence of C. trachomatis in young women.

The team gave some 13,000 female Army recruits a urine test that relies on a polymerase chain reaction to home in on and make copies of the DNA of C. trachomatis. The scientists found that nearly one out of 10 recruits was infected with chlamydia. The disease was particularly prevalent in recruits from the southern states. The team details its findings in the September 10 New England Journal of Medicine.

Another study by the same group discovered an alarmingly high rate of chlamydia infection among teenagers in Baltimore. The researchers tested more than 3,000 females, ages 12 through 19, who said they were sexually active. Some of the subjects had visited a family planning clinic, and others had gone to the school nurse for an unrelated health concern. None had symptoms of any sexually transmitted disease.

Twenty-four percent of the teens had a positive urine test for chlamydia. Sexually active 14-year-olds had the highest chlamydia rates. The team reports its findings in the Aug. 12 Journal of the American Medical Association. Quinn urges regular chlamydia testing for all sexually active teenagers. Without such a test, this silent disease may lead to an epidemic of pelvic inflammatory disease and infertility in years to come, he says.

Unlike those fighting syphilis, public health researchers trying to control the burgeoning chlamydia infections are nowhere near the point where they can talk about eliminating the bacterium. "We've got a long way to go," Quinn says.