Thousands of women risk infertility because Mycoplasma genitalium (MG) is developing resistance to antibiotics, according to sexual health specialists who are urging doctors to test before treatment.
Mycoplasma genitalium (MG) often has no symptoms but can cause pelvic inflammatory disease, which can leave some women infertile.
Experts say a lack of test kits means it is regularly confused with chlamydia and treated with incorrect doses of antibiotics - building up unsafe antibiotic resistance which could see it soon become untreatable.
Much like chlamydia, many people don't experience any symptoms if they are infected, although it can cause discharge, bleeding, and a burning sensation while peeing.
The British Association of Sexual Health and HIV on Wednesday issued new NICE-approved guidelines to prevent an explosion of infection rates within 5 to 10 years.
There is also concern the STI may grow resistant to antibiotics if not treated correctly.
It's not a great time to be having unprotected sex.
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MG does not always cause symptoms and will not always need treatment, but it can be missed or mistaken for a different sexually transmitted infection, such as Chlamydia.
BASHH recommends that MG is treated with a seven-day course of the antibiotic, doxycycline, followed by a course of azithromycin.
Although tests for MG have been developed they are not now available at all clinics.
Dr Peter Greenhouse, a sexual consultant in Bristol and BASHH member, told the BBC, "it's yet another good reason to pack the condoms for the summer holidays - and actually use them".
Almost half of 16 to 24-year-olds admit they have had sex with a new partner without using a condom, a Public Health England report said in December.
Public Health England says testing is available to diagnose MG and any signs of drug resistance, if necessary.
"The new BASHH guideline on MG is a positive step forward to improving testing and diagnosis", said Helen Fifer, a consultant microbiologist at Public Health England.