Bacterial vaginosis is not officially classified as a sexually transmitted infection, but the relationship between BV and sex is more complicated than a simple yes or no. BV is classified as a “vaginal dysbiosis,” meaning it’s an imbalance in the bacteria that naturally live in the vagina rather than an infection caused by a specific pathogen passed between partners. Yet it rarely occurs in people who have never had sex, and sexual activity is one of the strongest risk factors for developing it.
That gray area is exactly why this question comes up so often. The answer matters because it shapes how you think about prevention, whether your partner needs treatment, and how seriously you take recurrences.
Why BV Isn’t Classified as an STI
A textbook STI is caused by a specific organism, like chlamydia or gonorrhea, that passes from one person to another during sex. BV doesn’t work that way. Instead of one culprit, BV involves the healthy bacteria in the vagina (primarily lactobacillus species that produce lactic acid and hydrogen peroxide) being displaced by a surge of anaerobic bacteria. Several different species are involved, and no single sexually transmitted pathogen has been identified as the trigger.
The CDC describes BV as a vaginal dysbiosis and notes that “the cause of the microbial alteration that precipitates BV is not fully understood.” In other words, scientists know what the end result looks like (too many harmful bacteria, too few protective ones) but can’t definitively explain what kicks off the chain reaction in every case. That uncertainty is one reason BV hasn’t been reclassified as an STI despite its strong link to sexual activity.
Why It Behaves Like One
Even though BV falls outside the official STI category, the patterns look suspicious. The CDC acknowledges that BV “most often occurs in those who are sexually active” and “rarely affects those who have never had sex.” Those two facts alone explain why many researchers and clinicians treat the distinction as increasingly blurry.
One key piece of the puzzle is biofilm. The bacteria most associated with BV can form a structured community that clings to vaginal tissue and shields itself inside a self-made protective layer of carbohydrates, proteins, and nucleic acids. This biofilm can harbor multiple BV-associated species, making the imbalance stubbornly persistent. Research suggests that bacteria within this biofilm can be shared between sexual partners, which helps explain why BV so frequently recurs after treatment.
The evidence for partner-to-partner exchange has grown strong enough that in 2025, the American College of Obstetricians and Gynecologists (ACOG) recommended concurrent sexual partner treatment for recurrent BV for the first time. ACOG noted that previous data hadn’t demonstrated a clear benefit, but new research on male sexual partner therapy and a growing body of evidence implicating sexual activity as a risk factor changed that calculus.
What Actually Triggers BV
Sexual activity is the most consistent risk factor, but it isn’t the only one. Anything that disrupts the vaginal microbiome can tip the balance. Douching is a well-documented trigger because it washes away the protective lactobacillus bacteria. Antibiotic use for unrelated infections can do the same thing. A new sexual partner, multiple partners, or sex without condoms all raise the likelihood.
The condition is also notably common among women who have sex with women, likely because BV-associated bacteria can be shared through direct genital contact or shared toys. This pattern further supports the idea that sexual transmission plays a real role, even if BV doesn’t fit neatly into the STI box.
Symptoms and How It’s Diagnosed
About half of people with BV don’t notice any symptoms at all. When symptoms do appear, the most recognizable one is a thin, grayish-white vaginal discharge with a distinctive fishy smell, often stronger after sex. You might also notice mild itching or burning, though BV is generally less irritating than a yeast infection.
Diagnosis typically involves a clinical exam. Your provider checks for a few specific signs: elevated vaginal pH (above the normal acidic range), the characteristic discharge, a fishy odor when a chemical solution is added to a sample, and the presence of “clue cells,” which are vaginal cells coated in bacteria visible under a microscope. Meeting three of these four criteria confirms the diagnosis.
Why BV Matters Beyond Discomfort
BV is easy to dismiss as a minor nuisance, but untreated, it raises the stakes for several more serious health concerns. Having BV increases susceptibility to other STIs, including HIV. A large prospective study of African couples found that BV was associated with a 60% increased risk of HIV acquisition in women. Even more striking, men whose female partners had BV were more than three times as likely to contract HIV compared to men whose partners had normal vaginal flora, even after controlling for other risk factors like sexual behavior and circumcision.
BV also increases vulnerability to chlamydia, gonorrhea, and herpes. The mechanism makes intuitive sense: the protective lactobacillus bacteria that BV displaces are the vagina’s first line of immune defense. When that barrier is weakened, other pathogens have an easier entry point.
During pregnancy, BV has been linked to a higher chance of preterm delivery, though the U.S. Preventive Services Task Force notes it remains unclear whether BV directly causes preterm birth or is a marker for other underlying risks.
Treatment and the Recurrence Problem
BV is treated with prescription antibiotics, either taken orally or applied as a vaginal gel or cream. Treatment typically lasts five to seven days and clears the infection in most cases. The frustrating reality, though, is that BV comes back frequently. Recurrence rates within a few months of treatment are high, and this is where the sexual transmission question becomes very practical.
If BV-associated bacteria are harbored by a sexual partner and reintroduced after treatment, it explains why the same person keeps getting reinfected. This is exactly the reasoning behind ACOG’s 2025 recommendation to treat sexual partners when BV keeps recurring. If you’re dealing with repeated episodes, it’s worth discussing concurrent partner treatment with your provider, since this approach is now supported by clinical evidence where it previously wasn’t.
Using condoms consistently, avoiding douching, and limiting products that contact the vaginal area (scented soaps, sprays, washes) can all help maintain a healthier bacterial balance and reduce recurrence risk.
The Bottom Line on Classification
BV occupies a genuinely ambiguous space. It is not an STI by current medical classification because no single transmitted organism causes it. But sexual activity is its primary risk factor, bacteria associated with BV can be shared between partners, and treating partners now appears to reduce recurrences. For practical purposes, treating BV as sexually associated, even if not sexually transmitted in the traditional sense, gives you the most useful framework for preventing and managing it.