As if there weren’t already enough reasons to practise safe sex, a delightful new study has indicated that huge numbers of people may be infected with a rather nasty sexually transmitted illness, and know absolutely nothing about it.

The cause of the condition is a bacterium called Mycoplasma genitalium (MG), which, as the name suggests, has a propensity for colonizing the reproductive tracts of both men and women. Despite being the smallest free-living cell in the world, this little germ packs quite a punch, and can cause symptoms such as acute urethritis in men, which involves inflammation of the urethra, and cervicitis in women – inflammation of the uterine cervix. A range of other nasty surprises such as genital discharge and bleeding after sex (particularly for women) are also in its repertoire of effects. Although more data is needed, some studies have suggested that if left untreated, it could lead to female infertility. 

However, in a recent study that appeared in the International Journal of Epidemiology, 94.4 percent of infected men and 56.2 percent of infected women reported no symptoms, and therefore had absolutely no idea that they were afflicted with the condition.

While MG has been known about since the 1980s, it has still not been fully confirmed whether or not the condition is sexually transmitted. The present study lends yet more credence to the growing evidence that this is indeed the case, since it found that MG infections were strongly associated with sexual risk-taking. For instance, the illness was most prevalent in those who have had multiple partners or practiced unsafe sex. In contrast, no infections were detected in those who had never had sex.

Of the 4,507 participants in the study, 1.2 percent of men and 1.3 percent of women were found to carry MG, yet the majority of these had never experienced any symptoms. Fortunately, the condition is easily treated with a single course of antibiotics, yet the fact that so many people do not realize they carry the disease means they are unlikely to seek treatment before passing it on.

The research was conducted by scientists at University College London and Public Health England (PHE), and comes at a time when STIs are on the rise in the U.K. capital. A recent report published by PHE has revealed that the number of STI diagnoses in London rose by five percent in 2014 compared to the previous year. Overall, cases of such illnesses are 65 percent higher in London than in any other region of the U.K.