For the last few decades, syphilis has been re-emerging around the world. This may seem surprising; it can be treated successfully with antibiotics, after all. Yet, amid failures in public health campaigns and the challenge of fighting a disease that can sometimes spread undetected, syphilis is making its comeback. Estimates suggest that there were 10.6 million cases worldwide in 2008. And with its reemergence, it brings a new weapon: drug resistance.

For Natasha Arora, a population geneticist, the reemergence was intriguing. Although she started out studying orangutans, the tools of population genetics are multi-purpose, and she took a sideways step into researching syphilis instead. By studying syphilis samples taken from patients around the world, Arora and a team of researchers have found that one of the two modern strains of syphilis is far more widespread than previously thought and that both current strains show resistance to second-line antibiotics called macrolides.

Penicillin is the first choice to treat syphilis, but sometimes macrolides are prescribed instead. “Penicillin needs to be given intramuscularly (for early syphilis) or intravenously (for late syphilis),” says Lola Stamm, who sent syphilis samples to Arora and her team but wasn’t an author on their paper. “This can be a problem in developing countries, and most people don’t like needles.”