For the first time in three decades, the nation's most common sexually transmitted disease is a little less common. However, rates for a less common but highly contagious STD rose significantly, according to an annual report released Tuesday by the U.S. Centers for Disease Control Prevention.

Health officials said cases of chlamydia fell last year by 1.5 percent. But there were still 1.4 million new cases reported -- more than any other sexually transmitted disease. Experts are not sure what to make of the one-year decline.

A chlamydia infection is easily cleared with antibiotics. However, if left untreated the infection can damage a woman's reproductive system. A woman may have no symptoms, which is why chlamydia is sometimes called called a "silent" infection.

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In a worrisome development, the report also found rates of the most contagious forms of syphilis rose about 10 percent. There were a total of 17,375 reported cases of primary and secondary syphilis in 2013, which was seen mainly among gay men. (Primary and secondary refer to the earliest stages of the disease.) Men who have sex with men accounted for 75 percent of diagnosed syphilis cases in 2013.

If caught early, syphilis can also be treated with antibiotics. However, there are a number of long-term complications when primary and secondary syphilis is not treated. The infection can lead to blindness, paralysis and stroke, and also increase one's risk for HIV.

The CDC also reported gonorrhea cases declined, by less than 1 percent. In recent years, this STD has made headlines with the emergence of an increasing number of cases of drug-resistant gonorrhea.

The agency said early screening and diagnosis is always essential to prevent complications and transmission of STDs to sexual partners. It recommends sexually-active women under 25 request annual screening for chlamydia and gonorrhea. Pregnant women should request screening for all STDs, as well as HIV and hepatitis B. The agency recommends sexually-active gay and bisexual men undergo comprehensive STD screening annually. Safe-sex practices and condom use can significantly lower a person's risk for STDs.