The human throat houses billions of bacteria, most of them harmless. But one species is becoming more common, and it is anything but benign.

Drug-resistant gonorrhea has been on the rise for years; the World Health Organization has reported an increase in more than 50 countries. Now scientists say the epidemic is being driven by a particular mode of transmission: oral sex.

“The throat infections act as a silent reservoir,” said Emilie Alirol, the head of the sexually transmitted infections program at the Global Antibiotics Research and Development Partnership. “Transmission is very efficient from someone who has gonorrhea in their throat to their partner via oral sex.”

Oral gonorrhea is hard to detect and treat. Even more worrisome, these bacteria pick up resistance to antibiotics directly from other bacteria in the throat — and then are communicated to sex partners.