Bacterial vaginosis is not officially classified as a sexually transmitted infection, but sexual activity is one of the strongest risk factors for developing it. The reality is more nuanced than a simple yes or no. BV rarely affects people who have never had sex, and the bacteria linked to it can be shared between partners. The current scientific understanding places BV somewhere between a classic STI and a condition that simply coincides with sexual activity.
Why BV Isn’t Classified as an STI
A traditional STI has a clear-cut cause: one specific pathogen passes from person to person during sex. BV doesn’t work that way. Instead of a single germ causing the infection, BV develops when the balance of bacteria in the vagina shifts. Healthy vaginal flora is dominated by Lactobacillus species, which produce lactic acid and hydrogen peroxide to keep harmful bacteria in check. When those protective bacteria decline, anaerobic bacteria (the kind that thrive without oxygen) multiply and take over. That overgrowth is what produces the symptoms of BV.
Because researchers can’t point to one organism as “the cause” and because BV occasionally develops in people with no sexual history at all, the CDC describes it as a common vaginal condition rather than an STI. But that label is increasingly seen as incomplete.
The Evidence That Sex Plays a Central Role
Multiple lines of evidence point to sexual transmission as a major driver of BV, even if the classification hasn’t caught up.
Unprotected sex disrupts the vaginal environment directly. The presence of semen in vaginal fluid has been linked to more than double the risk of BV and recurrent BV, along with lower concentrations of protective Lactobacillus bacteria. Semen is alkaline, and the vagina’s natural acidity is one of its main defenses. Repeated exposure without a barrier shifts the pH upward, creating conditions where anaerobic bacteria can flourish.
Studies of female sexual partners provide some of the most compelling data. In one study of monogamous lesbian couples, 73% of women whose partner had BV also had BV themselves. Among couples where the index partner did not have BV, only 10% of their partners tested positive. The likelihood of a partner having BV was nearly 20 times greater when the other partner was affected. That pattern is hard to explain without sexual transmission.
The bacteria most closely associated with BV, particularly Gardnerella vaginalis, also show up in male partners at notable rates. In a study of over 400 symptomatic men tested with sensitive DNA methods, Gardnerella was the single most commonly detected organism, found in about 22% of patients. Researchers have proposed a model in which Gardnerella is sexually transmitted, then adheres to vaginal tissue and builds a protective biofilm community that displaces Lactobacillus and recruits other anaerobic bacteria.
Why BV Keeps Coming Back
One of the most frustrating features of BV is its recurrence rate. Up to 69% of women experience a return of symptoms within 12 months of completing standard treatment. For years, clinical guidelines recommended treating only the person with symptoms, not their sexual partner. The logic followed the “BV isn’t an STI” classification: if it’s not transmitted, there’s no reason to treat the other person.
That thinking is shifting. In October 2025, the American College of Obstetricians and Gynecologists recommended concurrent partner treatment for recurrent BV for the first time. The rationale is straightforward: if BV-associated bacteria live in a male partner’s urethra or on shared sex toys, treating only one person allows reinfection to happen the next time the couple has sex. The high recurrence rate itself has become one of the strongest indirect arguments that sexual transmission matters.
Risk Factors Beyond Sex
Sexual activity isn’t the only thing that can tip the vaginal microbiome out of balance. Douching is one of the best-documented non-sexual risk factors, as it physically washes away protective bacteria and disrupts pH. Having multiple sexual partners increases risk, likely because each new partner introduces a different microbial community. Other contributing factors include antibiotic use (which can kill Lactobacillus along with whatever infection it’s targeting), hormonal changes, and smoking.
That said, the CDC is clear that BV most often occurs in people who are sexually active, and it rarely develops in those who have never had sex. The non-sexual triggers exist, but they account for a minority of cases.
How BV Is Diagnosed
BV is diagnosed based on a combination of physical signs rather than a single test. Clinicians look for at least three of the following four criteria: a thin, milky discharge that coats the vaginal walls; a vaginal pH above 4.5 (higher than the normal acidic range); a fishy odor from the discharge; and the presence of “clue cells” under a microscope, which are vaginal cells covered in a visible layer of bacteria. Many people with BV notice the odor first, particularly after sex, because semen’s alkalinity releases the compounds responsible for the smell.
What Reduces Your Risk
Consistent condom use is one of the most effective ways to lower BV risk. Women who used condoms for every sexual encounter had roughly 45% lower odds of developing BV compared to those who used them inconsistently or not at all. When the analysis was narrowed to exclude borderline cases, the protective effect was even stronger, with a 63% reduction in risk. Condoms likely help by preventing semen from raising vaginal pH and by reducing the transfer of BV-associated bacteria from a partner.
Avoiding douching is equally important. The vagina is self-cleaning, and introducing water, vinegar, or commercial douching products strips away the Lactobacillus bacteria that maintain a healthy environment. Limiting the number of sexual partners also lowers exposure to unfamiliar bacterial communities, though this is a statistical relationship rather than a guarantee.
What This Means for You and Your Partner
If you have recurrent BV, the emerging evidence strongly supports having a conversation with your partner about concurrent treatment. This is especially relevant for women in relationships with male partners, where Gardnerella and other BV-associated bacteria can persist in the urethra without causing any symptoms the male partner would notice. For female couples, the high concordance rates between partners suggest that treating both people at the same time may help break the cycle of reinfection.
The bottom line: BV behaves like a sexually transmitted condition in most practical respects, even though it doesn’t carry the official STI label. Sex is the primary way the associated bacteria are exchanged, condoms significantly reduce risk, and treating partners appears to reduce recurrence. Whether or not the classification eventually changes, the prevention strategies look the same.