*(Via MSN News) – It’s hard to miss “Syphilis Explosion” ads that prominently feature those capitalized words before a fiery red erupting volcano. They were plastered across BART stations in the Bay Area and billboards in Los Angeles late last year courtesy of the AIDS Healthcare Foundation, which says the campaign has already appeared in or is heading to Cleveland, Columbus, Baton Rouge, Brooklyn and other cities.

Syphilis, which can cause serious health problems if untreated, is on a troubling upswing across the United States. The rate of primary and secondary syphilis infections fell by nearly 90 percent between 1990 and 2000, but has increased most years since, according to a Centers for Disease Control and Prevention report analyzing public health data through 2013. This recent wave of syphilis is mostly a problem confronting gay men—or, to use the broader medical term, men who have sex with men. Between 2000 and 2004 alone, the CDC reports that the estimated proportion of primary and secondary syphilis infections attributable to men having sex with men rose dramatically, from 7 percent to 64 percent.

Nationwide, the CDC reports that primary and secondary syphilis rates increased by 10 percent between 2012 and 2013—an infection rate more than twice as high as figures from 2001. The Atlanta, Baltimore, Detroit, Los Angles, Miami, Orlando, Portland, San Antonio, San Diego and San Francisco metro areas have some of the highest syphilis rates, according to the CDC.

This story from City Lab continues at MSN News.