Bacterial vaginosis (BV) is not classified as a sexually transmitted infection, but sex is one of the strongest risk factors for developing it. That distinction matters more than it might seem. BV doesn’t behave like chlamydia or gonorrhea, where a specific pathogen passes from one person to another. Instead, it’s a shift in the vaginal microbiome, where the protective bacteria that normally dominate get replaced by a mix of anaerobic bacteria that thrive in less acidic conditions. Sexual activity can trigger that shift, but it’s not the only cause.
Why BV Isn’t Classified as an STI
Traditional STIs involve a specific organism that transmits between partners during sex. BV doesn’t work that way. It involves an overgrowth of bacteria that are already present in the vagina, including species like Gardnerella vaginalis, Prevotella, and Mobiluncus. When levels of Lactobacillus (the bacteria responsible for keeping the vagina acidic and protected) drop, these other species multiply and take over. The result is a condition called vaginal dysbiosis rather than a new infection from an outside source.
The CDC notes that whether BV results from acquiring a single sexually transmitted pathogen is still unknown. Women who have never been sexually active rarely develop BV, which strongly links the condition to sexual activity. But because no single transmissible organism has been identified as the cause, it sits in a gray zone: sexually associated, but not sexually transmitted in the traditional sense.
How Sex Disrupts the Vaginal Microbiome
The healthy vagina maintains a pH below 4.5, which is acidic enough to suppress the growth of harmful bacteria. Semen is alkaline, with a pH around 7.2 to 8.0. When semen enters the vagina, it raises the pH, which alters bacterial growth patterns. Research published in The Journal of Sexual Medicine found that factors in semen directly contribute to the growth of pathogenic bacteria while inhibiting the Lactobacillus species that serve as the vagina’s natural defense. This doesn’t mean sex causes BV every time, but it explains why unprotected intercourse is a consistent risk factor.
Beyond the chemistry, the physical introduction of new bacteria from a partner’s body can further disrupt the microbial balance. Having a new sex partner, multiple partners, or inconsistent condom use all increase risk. Each of these scenarios introduces different microbial communities to the vagina, making it harder for Lactobacillus to maintain dominance.
BV Between Female Partners
Some of the strongest evidence for sexual transmission comes from studies of women who have sex with women. Female partners frequently share the same vaginal bacteria, and if one partner has BV, the other is more likely to develop it too. Cleveland Clinic notes that while BV doesn’t spread from person to person in the classic sense, you can get BV from a female partner, and bacteria associated with BV can be passed to another female partner.
This pattern suggests that BV-associated bacteria can transfer between women through shared vaginal contact, sex toys, or oral sex. If you have a female partner and you’re diagnosed with BV, letting them know is important so they can be evaluated and treated if needed.
Can Men Carry BV Bacteria?
Men can’t develop bacterial vaginosis because they don’t have a vaginal microbiome. However, BV-associated bacteria can live on the penis, particularly under the foreskin. This means a male partner can carry the bacteria responsible for BV and reintroduce them during sex, even after a woman has been treated. This cycle of reinfection is one reason BV recurs so frequently.
In a significant shift, the American College of Obstetricians and Gynecologists (ACOG) now recommends considering concurrent treatment for male sexual partners of women with recurrent BV. The recommendation follows growing evidence that treating only the woman often isn’t enough. Up to 66% of women experience a recurrence within a year of their initial treatment, and partner treatment offers a way to break that cycle. Male partners may be prescribed a combination of oral and topical antimicrobial agents alongside their partner’s treatment.
Other Risk Factors Beyond Sex
Sexual activity isn’t the only thing that disrupts vaginal bacteria. Douching is one of the most well-documented risk factors. It physically flushes out Lactobacillus and raises vaginal pH, creating the exact conditions that allow BV-associated bacteria to flourish. The Mayo Clinic is blunt about it: douching upsets the vagina’s healthy balance and can directly lead to an overgrowth of anaerobic bacteria.
Other factors that increase risk include:
- Smoking: associated with lower Lactobacillus levels in the vagina
- Antibiotic use: can kill off protective bacteria along with harmful ones
- IUD use: some studies link intrauterine devices to a slightly higher BV risk
- Hormonal changes: shifts in estrogen levels affect the vaginal environment
Using latex condoms or dental dams during sex reduces exposure to semen and limits the exchange of bacteria between partners. While no method eliminates BV risk entirely, consistent barrier use is one of the most practical steps you can take.
Why This Matters for Your Health
BV is often dismissed as a minor nuisance, but it carries real health consequences. The characteristic thin, grayish-white discharge and fishy odor are the most recognizable symptoms, though many women with BV have no symptoms at all. Left untreated, BV increases vulnerability to other infections. A meta-analysis published in the journal AIDS found that BV raises the risk of acquiring HIV by approximately 60%. It also increases susceptibility to other STIs and can lead to complications during pregnancy, including preterm birth.
Diagnosis typically involves a clinical exam where a provider checks vaginal pH (above 4.5 is a key marker), looks for clue cells under a microscope, and assesses the quality of discharge. A positive result on at least three of these indicators confirms BV. Treatment is straightforward and usually involves a course of antibiotics, but the high recurrence rate means that addressing contributing factors, including partner treatment when appropriate, is just as important as the initial prescription.
The Bottom Line on Transmission
BV occupies an unusual space. It’s not an STI in the way most people understand that term, yet sexual activity is deeply intertwined with how it develops and recurs. The bacteria involved aren’t foreign invaders passed from partner to partner like gonorrhea. They’re residents of the vaginal ecosystem that overgrow when conditions change. Sex is one of the most common triggers for that change, whether through the pH effects of semen, the introduction of new bacteria from a partner, or the transfer of BV-associated organisms between female partners. Treating it effectively often means thinking about it as a condition shaped by sexual partnerships, even if it doesn’t fit neatly into the STI category.