Government health officials say there were more than 1 million cases of chlamydia reported in the United States last year.
A doctor with the Centers for Disease Control and Prevention calls that number "a new U.S. record" for any sexually transmitted disease in a single year.
Other sexually transmitted diseases, including gonorrhea and syphilis, are also on the increase. Researchers say they are also seeing growing numbers of gonorrhea cases caused by antibiotic-resistant bacteria.
In 2004, the CDC reported 112 cases of gonorrhea per 100,000 people, the lowest infection rate since 1941. In 2006, the number went to about 121 per 100,000.
Fewer than 10,000 syphilis cases were reported last year, but the rates still climbed 14 percent from 2005.