Sexually transmitted diseases are on the rise in the United States. Last year, 2.3 million cases of chlamydia, gonorrhea and syphilis were diagnosed — 200,000 more than the previous record number, from 2016, according to a report from the Centers for Disease Control and Prevention. In fact, the CDC director said, the increase has been “steep and steady” for the past four years, calling it “a persistent and troubling trend.” The most common STD has been chlamydia, with 1.7 million diagnoses last year, with nearly half of those cases in women and girls ages 15 to 24. All three diseases are bacterial infections that usually can be cured with antibiotics. However, the CDC says, most cases go undiagnosed and untreated. People may not have symptoms, or they disregard what symptoms they do have. Left untreated, chlamydia can damage the female reproductive system, making pregnancy difficult or impossible. Untreated gonorrhea can cause infertility and increase the risk of getting HIV/AIDS. Syphilis that is not treated can lead to heart or brain damage and can be life-threatening. Because these diseases are spread through sexual contact, the best prevention is use of a condom.