Bacterial Vaginosis

Routine gynecological examinations are part of Candace Cooley’s experience, and bacterial vaginosis is a condition those visits regularly bring to light. Its reach is wide. The University of Minnesota news service CIDRAP describes it as a disruption of the vaginal microbiome that affects nearly one in three women worldwide, and reports that more than half of women see it return within three months of antibiotic treatment.¹ In the United States, the Centers for Disease Control and Prevention calls it the commonest vaginal condition in women aged 15 to 44.⁵ Those two facts, that it is common and that it comes back, frame almost everything researchers have asked about it.

A shift in the vaginal microbiome

Bacterial vaginosis, usually shortened to BV, is not an infection with one invading germ. The CDC defines it as an imbalance that develops when there is too much of certain bacteria in the vagina.⁵ The healthy pattern is unusual by the standards of the body. In an NPR interview on the vaginal microbiome, the microbiologist Fatima Aysha Hussain explained that a microbiome linked with good health has low diversity, dominated by lactobacilli, especially one species, which make lactic acid and keep the environment acidic enough that few other organisms thrive.⁹ In BV that order breaks down into a diverse mix of anaerobic bacteria, the opposite of the gut, where diversity is generally a good sign.⁹

Many people with BV notice nothing. When symptoms do appear, the CDC lists a thin white or gray discharge, pain, itching or burning in the vagina, a strong fishlike odor that is often worse after sex, burning when urinating, and itching around the outside of the vagina.⁵ A clinician can examine for discharge and test a sample of vaginal fluid to confirm it.⁵

The CDC notes that douching, not using condoms and having new or multiple sex partners can upset the balance of vaginal bacteria; that BV rarely affects people who have never had sex; and that it cannot be caught from toilet seats, bedding or swimming pools.⁵ It matters beyond discomfort. The agency links it with a higher chance of acquiring other sexually transmitted infections, including HIV, and with babies born early or small when it occurs in pregnancy.⁵ Untreated, it can raise the chance of other infections such as chlamydia and gonorrhea, which can in turn cause pelvic inflammatory disease.⁵ Antibiotics are the standard treatment, and the CDC adds that BV can return even after a full course.⁵

Figure

Recurrence within 12 weeks in the 2025 partner trial

• Woman and male partner treated35
• Woman treated alone63

percent

The suspicion that sex carries BV is old. Hussain recounted that in the 1950s and 1960s the physician whose name survives in Gardnerella vaginalis, one of the bacteria associated with BV, transferred vaginal fluid from women with the condition to healthy women and produced it in them, an experiment she called very unethical.⁹ Yet treating partners did not seem to help. In a double blind Danish trial of 107 couples, men were given either the same antibiotic as their partners or a placebo, and treating the man made no difference to the women’s symptoms, signs or test results five weeks later.⁴ For decades the guidance followed that evidence. The CDC page, last reviewed in December 2023, still says that male partners of women with BV do not need treatment, though it can pass between female partners.⁵

Recurrence and the partner question

That position was tested in a trial run in Australia from April 2019 to November 2023 and published in March 2025. It enrolled couples in which the woman had BV and was in a monogamous relationship with a man. In both groups the woman received the recommended first line treatment. In one group the man also took an oral antibiotic and applied an antibiotic cream to the skin of the penis for seven days; in the other he received nothing, which was standard care.² A safety board stopped the trial early, after 150 couples had finished 12 weeks of follow up, because treating the woman alone was clearly inferior. BV returned in 35 percent of women whose partners were treated and in 63 percent of those whose partners were not.² HealthDay reported that the treated men’s side effects were nausea, headache and a metallic taste.³

Why had earlier trials failed? Catriona Bradshaw, a lead investigator, told CIDRAP that those studies had design limits and that none had combined oral and topical antibiotics to clear BV bacteria from men, especially from the skin of the penis.¹ An accompanying editorial called the result a paradigm shift and urged clinicians to begin discussing sexual transmission with patients. Its authors also asked for similar trials in more diverse populations, since racial and ethnic differences in BV are well documented, and in women without a regular male partner.¹ Coverage in The Guardian set out the wider meaning: BV had long been thought to fall short of the definition of a sexually transmitted infection, partly because of the earlier negative trials, and the new data challenged that view. One sexual health consultant welcomed the paper but noted that some women who stopped having sex still found BV hard to clear, so partner treatment could not be the whole answer.⁶

Figure

Why the 2025 partner trial differed

	Earlier partner trials	The 2025 trial
Treatment for men	Oral antibiotics only	Oral antibiotics plus a cream applied to the skin
Aim	Treat bacteria carried by the man	Also clear bacteria from the skin of the penis
Result	No gain in cure for the women	Recurrence cut by about half; the trial was stopped early
How it was read	As evidence against sexual transmission	As support for treating BV as sexually transmitted

The next questions concerned the women who were not cured. Interviews with trial participants, described by the University of Melbourne, explored what would make couples take up the approach, and fed into new resources for patients, partners and clinicians.⁷ In October 2026 The Guardian reported a follow up by the same group, involving 188 couples, which set out to explain why about one in five women treated together with their partners still had BV. It found two distinct routes back: persistence of the original imbalance despite a week of antibiotics, and reinfection. About two thirds of women had a healthy, lactobacillus rich microbiome the day after finishing treatment; the other third appeared not to have cleared BV and were nearly three times as likely to have it again within four weeks.⁸ The researchers plan trials of longer courses and combinations of antibiotics, and they cautioned that a study whose participants reflected urban Australia might not apply everywhere, including low and middle income countries and other settings where BV weighs heavily.⁸

Figure

Two routes back to bacterial vaginosis

1. 01
   PersistenceAbout a third of women had microbiomes suggesting the infection had not cleared before sex resumed, and they were nearly three times as likely to have it again within four weeks.
2. 02
   ReinfectionBacteria passed back from a partner, the route that partner treatment is meant to block.
3. 03
   Next trialsLonger courses and combinations of antibiotics aimed at the women whose infection persists.

Other groups are testing whether the healthy community itself can be restored. In the NPR interview, Hussain described a small safety pilot of vaginal microbiota transplant, in which fluid from a carefully screened donor is placed in a recipient after antibiotics. Two of the four women who received it shifted to a lactobacillus dominated microbiome that lasted up to six months, and two did not.⁹ Treatment decisions for any particular woman, including whether a partner should be involved, belong with her own clinician.

BV also shows how much ordinary gynecology still rests on questions that went unasked for decades, and how often a common complaint turns out to have a mechanism worth studying. The next entry turns from microbes to hormones and mood, and to another common condition long dismissed as part of the monthly cycle: premenstrual dysphoric disorder. Related pelvic conditions with long histories of delay are covered under endometriosis and adenomyosis.