According to CBS News, instances of chlamydia fell by 1.5 percent last year for the first time in three decades. However, with 1.4 million new cases reported, it is still the most common STD. By contrast, occurrences of the highly contagious syphilis rose by 10 percent, primarily among gay men. The CDC reports that gonorrhea cases have declined, but only minutely. Gonorrhea has made headlines in recent years after reports of a drug-resistant form of the disease.