The infection was discovered in 1981 but very few people - even doctors - knew about it until recently. It often has no symptoms at all, but if left untreated, it can lead to pelvic inflammatory disease, and ultimately infertility in women.
The British Association of Sexual Health and HIV (BASHH) believes 3,000 women a year could lose the ability to have children if the STI becomes untreatable.
It can be treated with antibiotics - but the infection is developing resistance to some of these drugs.
For women, it can cause pain or bleeding during sex, burning sensation when urinating. Macrolide resistance in the United Kingdom is estimated at about 40%, say the guidelines.
"MG is rapidly becoming the new "superbug": It's already increasingly resistant to most of the antibiotics we use to treat chlamydia and changes its pattern of resistance during treatment, so it's like trying to hit a moving target", said Dr. Peter Greenhouse, the lead sexual health clinician at Weston General Hospital's Weston Integrated Sexual Health Centre. "So people need to take precautions". "Everyone can protect themselves from STIs by consistently and correctly using condoms with new and casual partners".
"It's yet another good reason to pack the condoms for the summer holidays - and actually use them".
Dr Helen Fifer, consultant microbiologist at Public Health England, welcomed the guidelines, adding: "If you have symptoms of an STI, we recommend you get tested at your local sexual health clinic".
But this trend was reversed following declining concern about HIV, with the emergence of effective new antiviral treatments reducing the consequences of an HIV diagnosis from a death sentence to a treatable condition.