One in 25 participants in a representative sample of 12,548 Americans ages 18-26 were found to be infected with chlamydia in the first nationwide study of the STD's prevalence. Racial and ethnic disparities in infection rates were also found. Blacks were six times more likely to be infected than whites. And though screening guidelines from CDC and professional medical groups focus on young women, who are more likely to be infected, lead author William Miller said he was surprised to find the gap between men and women is not as wide as previously thought.

Chlamydia can be cured with a single dose of oral antibiotics. However, most infected people do not know they have chlamydia because they have no symptoms. Women, who are more likely to suffer the STD's complications, will continue to get infected in large numbers unless men are diagnosed and treated as well, said Miller, an infectious disease specialist and epidemiologist at University of North Carolina-Chapel Hill.

Even with the availability of a urine test, "men do not routinely go for preventive health care," said Jonathan Zenilman, an STD expert at The Johns Hopkins University. Zenilman said it is unlikely that local health departments could afford to widen chlamydia screenings. But since it became a quality-of-care measure, managed-care plans have increased testing of young women, he said.

Gonorrhea, which is also detected by the same test, was found in fewer than one in 200 participants.

The disparity between white and black infection rates is due to access to screening and demographics, Miller theorized. Blacks are less likely to get into the health-care system and are more likely to have sex with other blacks than whites, continuing the infection cycle, he said.

The chlamydia infection rate among men and women for the different racial groups was, respectively: 1.4 percent and 2.5 percent among whites; 7.2 percent and 4.4 percent among Latinos; 11.1 percent and 14 percent among blacks; 1.1 percent and 3.3 percent among Asian Americans; and 8 percent and 13.3 percent among Native Americans.

"Prevalence of Chlamydial and Gonococcal Infections Among Young Adults in the United States" was published in the Journal of the American Medical Association (2004;291(18):2229-2236).

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