Bacterial vaginosis (BV) happens when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment and keep it slightly acidic. When those protective bacteria decline and other types overgrow, the result is BV. It affects roughly 23% to 29% of women of reproductive age worldwide, making it the most common vaginal condition in that group.
BV is not caused by a single germ the way a cold or strep throat is. It’s a disruption of an entire ecosystem, which is why the triggers are varied and sometimes hard to pin down.
What Actually Changes Inside the Vagina
A healthy vaginal microbiome is dominated by lactobacilli, bacteria that produce lactic acid and hydrogen peroxide. That acid keeps the vaginal pH between about 3.8 and 4.5, which suppresses the growth of other organisms. When lactobacilli numbers drop, the pH rises above 4.5 and a diverse mix of anaerobic bacteria fills the gap. These include species of Gardnerella, Prevotella, Mobiluncus, Atopobium, and others. It’s this collective overgrowth, not any single species, that defines BV.
Gardnerella vaginalis tends to be present in the highest numbers and is the organism most closely associated with BV, but it can also exist in small amounts in a perfectly healthy vagina. The problem isn’t its presence. It’s the collapse of lactobacilli that lets it and dozens of other species multiply unchecked.
Sexual Activity and New Partners
Sex is one of the strongest and most consistent risk factors for BV. Having a new sexual partner, or multiple partners, increases the likelihood that the vaginal bacterial balance will be disrupted. The exact mechanism isn’t fully understood, but exposure to a partner’s genital bacteria can introduce organisms that compete with lactobacilli. Unprotected sex appears to carry more risk than sex with condoms.
For years, BV was not considered sexually transmitted in the traditional sense, and treating male partners wasn’t part of standard care. That changed in 2025 when the American College of Obstetricians and Gynecologists recommended, for the first time, that male sexual partners of women with recurrent BV be treated with antimicrobial therapy at the same time. This followed growing evidence that BV-associated bacteria can be harbored on the penis and reintroduced during sex, fueling the cycle of recurrence. The recommendation currently applies to women with recurring BV and male partners. More research is needed for same-sex partners and people in nonmonogamous relationships.
Women who have sex with women also have elevated BV rates, likely because the same bacteria can be shared between female partners.
Douching and Scented Products
Douching is one of the most well-documented triggers. Women who douche at least once a month have about a 1.4 times higher risk of developing BV. For women who had douched within the previous week, the risk jumps to 2.1 times higher. The brand of douche doesn’t matter; studies found similar odds regardless of the product used.
The vagina is self-cleaning. Douching flushes out the lactobacilli that maintain a healthy pH, effectively resetting the environment in favor of the bacteria you don’t want. Scented soaps, sprays, and bubble baths applied to the vaginal area can cause similar disruption, though the evidence is strongest for douching specifically.
Smoking
Cigarette smoking is an independent risk factor for BV, meaning it raises your risk even after accounting for sexual behavior and hygiene. Tobacco byproducts have been detected in vaginal secretions of smokers, and these chemicals appear to have an anti-estrogenic effect. Since estrogen supports the growth of lactobacilli, anything that suppresses estrogen activity can thin out that protective bacterial layer and make the vagina more hospitable to BV-associated organisms.
Hormonal Shifts
Estrogen plays a key role in maintaining a lactobacillus-friendly environment. The cells lining the vagina store glycogen, a sugar that lactobacilli feed on, and estrogen drives that glycogen production. When estrogen levels drop, the food supply for protective bacteria shrinks.
This is why BV risk can fluctuate with hormonal changes. Menstrual blood temporarily raises vaginal pH, and some women notice BV symptoms recurring around their period. Menopause brings a sustained drop in estrogen, which can shift the vaginal microbiome toward a less protective state. Pregnancy, despite being a high-estrogen state, also carries elevated BV risk for reasons that aren’t entirely clear but may involve immune system changes.
How BV Feels and How It’s Diagnosed
Many women with BV have no symptoms at all. When symptoms do appear, the most common is a thin, grayish-white discharge with a distinct fishy smell, often stronger after sex or during menstruation. Itching and irritation are possible but less typical than with a yeast infection.
Doctors diagnose BV using a combination of clinical signs. The standard approach looks for at least three of the following four: a thin, milklike discharge that coats the vaginal walls; a vaginal pH above 4.5; a fishy odor (sometimes tested by adding a chemical to a sample of discharge); and the presence of “clue cells,” which are vaginal cells covered in bacteria visible under a microscope. A lab-scored evaluation of a vaginal swab can also be used, grading the ratio of lactobacilli to other bacterial types on a scale from 0 to 10, with scores of 7 or higher indicating BV.
Why It Keeps Coming Back
Recurrence is the defining frustration of BV. Between 50% and 80% of women who complete a course of antibiotics will have BV return within 6 to 12 months. Standard treatment clears the overgrown bacteria but doesn’t always restore a robust lactobacillus population afterward, leaving the vagina vulnerable to the same imbalance.
Several factors feed this cycle. If a sexual partner carries BV-associated bacteria, reinfection can happen with every exposure. Ongoing douching or smoking keeps the environment unstable. And some women simply have vaginal microbiomes that don’t bounce back to a lactobacillus-dominant state easily after disruption. The new recommendation to treat male partners simultaneously is one of the first clinical strategies aimed specifically at breaking this recurrence loop. For women in long-term relationships with recurring BV, this is a meaningful shift in how the condition is managed.
Factors You Can and Can’t Control
Some BV risk factors are modifiable. Stopping douching, using condoms, and quitting smoking all reduce risk in measurable ways. Avoiding scented products in the vaginal area removes another potential source of disruption.
Other factors are harder to control. Your baseline vaginal microbiome composition is partly determined by genetics and ethnicity. Hormonal fluctuations tied to your menstrual cycle or life stage aren’t something you can simply override. And the reality that a new sexual relationship can trigger BV means that normal, healthy sexual behavior carries some inherent risk for this condition. BV is not a reflection of poor hygiene or reckless behavior. It’s an ecological shift in a complex microbial system, and it happens to millions of women every year.