Syphilis had become relatively rare in developed countries since the discovery of penicillin, though it continues to plague many parts of the global South. At the turn of the millennium, it looked as though the United States had all but eliminated the disease. After an outbreak in the early 1990s, diagnoses of primary and secondary syphilis had dropped to 2.1 cases per 100,000 people—the lowest rate since the U.S. began recording it in 1941. But in 2002, the downward trend began to reverse.

The new wave of syphilis shows no signs of slowing down. In New Orleans, the number of syphilis cases tripled between 2012 and 2014. Central New York, which two years ago reported 27 syphilis cases, most recently reported 110, and some health clinics are now offering free syphilis testing. Health officials in Oregon, where syphilis rates have increased by more than 1,000 percent from 2007 to 2014, have created a new website, syphaware.org. The site's homepage reads, “Oregon is known for many things: natural beauty, coffee, beer, and Pinot Noir. Did you know that Oregon is also known for syphilis?”

Researchers are still trying to work out why these increases are happening now, but the CDC’s report offers a few clues. For one, syphilis isn’t the only sexually transmitted disease becoming more common. Syphilis, chlamydia, and gonorrhea—the three STDs that comprised the focus of the report—rose simultaneously for the first time on record, which suggests an underlying cause that isn’t syphilis-specific.

Some health officials point to the growing role of technology in people’s sex lives, specifically apps like Tinder and Grindr that facilitate casual sex between partners who don’t know each other’s sexual histories. But there’s no conclusive evidence that these apps have played a role in syphilis outbreaks, especially given that Tinder was released more than a decade after syphilis rates began rising again in 2002.

Sarah Kidd, an epidemiologist at the CDC, believes dating apps can pose a diagnostic problem, since controlling the spread of syphilis relies on being able to notify an infected person’s sexual partners.“We do know that with the rise of so many apps, it's easier to meet partners and not necessarily have identifying information and not be able to track them down later,” she says.

Compared to chlamydia and gonorrhea, syphilis is particularly challenging from a public-health perspective because most Americans don’t think of it as a threat. The worst epidemics happened centuries ago, when the infection had no known cause or cure. In the early 16th century, the Italian physician Alexander Benedictus suggested that syphilis was spread through “a venereal taint produced in the sexual organs of women by the alteration of humors which they exhale.”

Across the world, people also blamed foreigners for bringing syphilis into their countries. In his book Opus Ultimum, Daniel N. Leeson writes: “The English called syphilis ‘The French Disease’; the French called it ‘The Italian Disease’; the Italians called it ‘The Turkish Disease’; the Russians called it ‘The Polish Disease’; and both the Japanese and the Indians termed it ‘The Portuguese Disease.’ Only the Spanish accepted any blame, referring to it as ‘The Spanish Disease.’” (They may have had good reason. Some historians believe that Columbus and his crew brought the infection from America upon returning to Europe, though that theory was challenged just last week when researchers found signs of congenital syphilis in the skeleton of a European child who died in the 14th century.)