Atlanta With more than 1 million cases of chlamydia last year, the U.S. set a record for sexually transmitted diseases.
But health officials believe better screening explains most of the increase - not an explosion of infections.
"The more we test, the more we find," said Dr. John M. Douglas Jr. of the Centers for Disease Control and Prevention.
The CDC released a new report that showed troubling rises not only in chlamydia but also in gonorrhea and syphilis.
CDC officials said they think only a fraction of cases are being detected and pushed for additional testing and treatment.
"If (health care) providers think young women in their practice don't have chlamydia, they should think again," said Dr. Stuart Berman, a CDC epidemiologist.
About 19 million cases of sexually transmitted infections occur each year in the United States, according to CDC estimates. The report released Tuesday focused on the three diseases caused by sexually transmitted bacteria, as opposed to viruses like HIV and herpes.
Chlamydia is the most common. Nearly 1,031,000 cases were reported last year, up from 976,000 the year before.
The count broke the single-year record for reported cases of a sexually transmitted disease, which was 1,013,436 cases of gonorrhea, set in 1978.
About three-quarters of women infected with chlamydia have no symptoms. Left untreated, the infection can spread and may lead to infertility. It's easily treated if caught early.
Health officials believe as many as 2.8 million new cases may be occurring each year, Douglas added.
In terms of rates, there were about 348 cases of chlamydia per 100,000 people in 2006, up 5.6 percent from 2005.
Chlamydia infection rates are highest in women in their teens and early 20s. They are also more than seven times higher in black women than whites, and more than twice as high in black women than Hispanics.
Chlamydia has been rising steadily since the mid-1990s, and health officials say several factors have been driving the increase.
One is improved reporting. A second is that urine and swab tests for the bacteria are getting better and are used more often. A third is a 1993 federal recommendation that pushed doctors to routinely test for the infection in sexually active women ages 15 to 25.
Despite the recommendation that young women get screened annually, health officials estimate only 40 percent do.
The report also found that gonorrhea rates are jumping again after hitting a record low, and an increasing number of cases are caused by a "superbug" version resistant to common antibiotics.
In 2004, the nation's gonorrhea rate fell to 112 cases per 100,000 people, the lowest level since the government started tracking cases in 1941.
But since then, health officials have seen two consecutive years of increases, and the rate in 2006 was about 121 per 100,000.
Health officials don't know exactly how many superbug cases there were among the more than 358,000 gonorrhea cases reported last year. But a surveillance project of 28 cities found that 14 percent were resistant to the standard antibiotic treatments.
The appearance of the superbug has been previously reported, and the CDC in April advised doctors to stop using those drugs against gonorrhea.
Douglas doesn't think the superbugs are the reason for gonorrhea's escalating numbers overall.
Other doctors are worried. The gonorrhea superbug has been on the rise not only in California and Hawaii, where the problem has been most noticeable, but also in the South and parts of the Midwest.
"Suddenly we're starting to see the spread," said Dr. Khalil Ghanem, an infectious disease specialist at Johns Hopkins University's School of Medicine.
Syphilis cases are rising, too, in both men and - more alarmingly - in women too, according to the report.