Gonorrhea infections are increasing, but doctors are running out of antibiotics that can fight the increasingly resistant bacteria causing the sexually-transmitted disease, the World Health Organization (WHO) warned on Friday.

Gonorrhea is caused by bacteria called Neisseria gonorrhoeae or gonococcus.

Experts said that incurable Gonorrhea has started to spread after becoming resistant to antibiotics, which has been partly caused by oral sex and a decline in condom use.

It is mainly found in discharge from the penis and in vaginal fluid and can easily pass between people through unprotected vaginal, oral or anal sex.

Two-thirds of the countries that report resistance data to the WHO have seen cases in recent years in which their antibiotics of last resort no longer worked against gonococci bacteria.

“These cases may just be the tip of the iceberg since systems to diagnose and report untreatable infections are lacking in lower-income countries where Gonorrhea is actually more common,” WHO expert, Teodora Wi, said in Geneva.

The UN health agency estimates that 78 million people are infected annually with the disease.

Britain and the U.S. reported increases of more than 10 per cent in 2015..

Cases among gay men in France doubled between 2013 and 2015.

Rates are highest in the African region, where one in 10 men is infected annually.

The main reasons for the increase are decreasing condom use, increased mobility as well as poor disease monitoring and inadequate treatment, according to the WHO.

Gonorrhea can infect the genitals, rectum and throat. It can lead to inflammation of the pelvis and to infertility.