But chlamydia is by far the most pervasive, ahead of gonorrhea and syphilis. Doctors diagnose about 3 million cases annually.

Oral contact can result in infection of the mouth and throat. A condom reduces the chances of transmission but is no guarantee. People with a history of unprotected sex and/or multiple sex partners should get screened.

Symptoms may appear within a week to three weeks of exposure.

Half of men never experience symptoms. The disease is easily curable if detected early. If it goes undetected, it can cause infertility in men and women.

About 10 to 15 percent of untreated women will develop pelvic inflammatory disease (PID), and up to 15 percent of women with PID will become infertile. Many women who develop PID will experience chronic pain, and PID can also lead to a life-threatening tubal pregnancy.

Chlamydia causes inflammation of the male reproductive system near the testicles.

One study found that women with chlamydia are six times more likely to develop cervical cancer and three to five times more likely to become HIV positive than women without the condition.

Both men and women can be screened via a simple urine test.