The rates of three leading sexually transmitted diseases -- chlamydia, gonorrhea and syphilis -- rose again in the United States last year, worried public health officials said on Tuesday.

It was the second year in a row of increases for all three of these sexually transmitted bacterial infections, the Centers for Disease Control and Prevention said.

The rate of chlamydia, the most common infectious disease reported to the CDC, increased 5.6 percent in 2006 from 2005.

More than 1.03 million chlamydia cases were reported in 2006 -- the highest number since the CDC began tracking it in 1984 and the highest number of annual U.S. cases ever for any sexually transmitted disease.

The CDC said the national rate of reported chlamydia cases in 2006 was 347.8 cases per 100,000 Americans. The rate has been rising since the 1980s but likely reflects less than half the actual occurrence, the CDC said.

The rate of gonorrhea rose 5.5 percent in 2006 -- its second straight yearly gain -- with 358,366 cases reported. In 2006, the gonorrhea rate was 120.9 cases per 100,000 people.

Rates for blacks were eight times higher than for whites for chlamydia and 18 times higher for gonorrhea.

The rate of syphilis, with gay and bisexual men accounting for 64 percent of cases, rose 13.8 percent in 2006 over 2005, with 9,756 cases. Syphilis rates have been rising over the past six years, gaining 57 percent since 2001, the CDC said.

The 2006 syphilis rate was 3.3 cases per 100,000 people.

Sexually transmitted diseases, or STDs, represent a significant public health threat, the CDC said, with about 19 million new infections occurring each year in a country of 300 million people -- almost half among people ages 15 to 24.

"This is a hidden epidemic," said Dr. Stuart Berman, who helps tracks STDs for the CDC.

"Most people are not aware of how many STDs are out there, the risks that they run and the need for getting regular testing for some of these and treatment -- and having their partners treated. We'd like to see these rates going down."

Unreported Diseases

The CDC said many STD cases go undiagnosed, and some common viral infections such as human papillomavirus -- which causes cervical cancer -- and genital herpes are not reported to the agency at all.

Dr. John Douglas, who heads CDC STD prevention efforts, said more risky sexual behavior by gay and bisexual men is helping fuel syphilis rates. He said expanded screening efforts may help explain the rise in chlamydia reports.

Having multiple sex partners and not using condoms can increase the risk of spreading sexually transmitted diseases, including syphilis and AIDS.

Douglas said STD prevention programs in local and state health departments have been stretched thin financially. And he said the lack of health insurance among many Americans might be contributing as well.

Chlamydia is easily cured with antibiotics, but often goes undiagnosed. When untreated, it can cause severe health consequences particularly for women such as pelvic inflammatory disease, ectopic pregnancy and infertility.

The CDC estimated there are about 2.8 million new cases of chlamydia among Americans each year, far higher than the reported number of cases. The rate for women is three times higher than for men.

STDs raise one's vulnerability to the AIDS virus and makes HIV-infected people more infectious. Left untreated, gonorrhea can cause a variety of health problems including infertility.

Syphilis, easily curable in its early stages, can cause serious complications if untreated, including brain, cardiovascular and organ damage, and even death.