1.3 million

Cases of chlamydia reported in 2010, a 5.1% increase since 2009 and the largest number of cases ever reported to the Centers for Disease Control and Prevention for any condition. The rise can be attributed to better screening, not to an actual increase in people with the disease. The number of new gonorrhea cases in 2010 was 309,341, an increase of 2.8% since 2009, following decades of decline. Overall cases of syphilis declined for the first time in a decade to 13,774; however, the rate rose slightly in men. Sexually transmitted infections still disproportionately affect blacks and gay men. [via CDC]