Gonorrhea, one of the most common sexually transmitted infections in the U.S., is getting harder and harder to treat — and unsafe oral sex is making the STI particularly dangerous.

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That's according to a warning issued Friday by experts at the World Health Organization, whose researchers examined data from patients with gonorrhea in 77 countries showed drug-resistant gonorrhea is getting harder (and sometimes impossible) to treat.

"The bacteria that cause gonorrhea are particularly smart," WHO medical officer Teodora Wi said in a news release. "Every time we use a new class of antibiotics to treat the infection, the bacteria evolve to resist them."

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An estimated 78 million people are infected with gonorrhea each year by unprotected vaginal, oral and anal sex. It disproportionately affects women.

Gonorrhea can infect the throat, genitals and rectum, but according to Wi, scientists are particularly concerned about the throat.

Wi told BBC that introducing gonorrhea bacteria into the throat through oral sex can lead to what's referred to as super-gonorrhea, a drug-resistant strain that is often untreatable.

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This happens because antibiotics taken to treat the infection mix with the super-gonorrhea in the throat and create resistance, Wi said.

The rise in resistant gonorrhea is largely due to decreased condom use, increased urbanization and travel, poor infection detection rates and either inadequate or failed treatment, according to the WHO.

Read the full WHO news release.