Dr. John M. Douglas Jr. of the Centers for Disease Control and Prevention in Atlanta said chlamydia, a sexually transmitted disease, can cause pelvic inflammatory disease in women, as well as atopic pregnancy and increase the risk for transmitting and developing HIV.

"STDs pose a serious and ongoing threat to millions of Americans," Douglas said at a teleconference. "Young women, racial and ethnic populations, and men who have sex with men are particularly hard-hit by these diseases -- STDs can have serious health consequences, particularly if they are undiagnosed and left untreated."

Chlamydia, the most common reportable U.S. infectious disease, rose 5.6 percent from 2005 to 347.8 cases per 100,000 people, Health Day News reported.

Gonorrhea dropped 74 percent from 1975 to 1997, however, gonorrhea rose 5.5. percent from 2005 with 358,366 new cases reported in 2006, the report found.

Gonorrhea is becoming resistant to some antibiotics, Douglas said.