Bacterial vaginosis (BV) is caused by a shift in the balance of bacteria in the vagina, where protective bacteria decline and harmful anaerobic bacteria overgrow. It’s the most common vaginal infection, affecting roughly 20 to 30% of women of reproductive age worldwide, with rates reaching 29% among women aged 14 to 49 in the United States. No single germ “causes” BV the way strep throat has one culprit. Instead, a combination of triggers can tip the vaginal environment out of balance.
How the Vaginal Environment Shifts
A healthy vagina is dominated by bacteria that produce lactic acid, keeping the pH below 4.5. That acidic environment, along with hydrogen peroxide and natural antimicrobial compounds, suppresses the growth of harmful organisms. When those protective bacteria drop in number, anaerobic species like Gardnerella, Prevotella, Atopobium, and Megasphaera move in. These bacteria produce short-chain fatty acids that raise vaginal pH and break down the mucosal proteins lining the vaginal walls, thinning the protective barrier.
One reason BV is so stubborn is that Gardnerella can build biofilms, structured communities of bacteria encased in a protective matrix of carbohydrates and proteins. Inside a biofilm, bacteria are far more resistant to the lactic acid and hydrogen peroxide that would normally keep them in check. Biofilms also make BV harder to treat. Standard antibiotics can reduce symptoms, but they often fail to fully eradicate the biofilm, which helps explain why up to 66% of women experience a recurrence within a year of treatment.
Sexual Activity and New Partners
Sexual activity is one of the strongest and most consistent risk factors. Having new or multiple sexual partners, or not using condoms, increases the likelihood of developing BV. Researchers still don’t fully understand the exact mechanism, but the exchange of bacteria between partners appears to disrupt vaginal flora. The bacteria involved in BV can be sexually transmitted and then alter the natural balance once established.
This connection is significant enough that in 2025, the American College of Obstetricians and Gynecologists recommended for the first time that male sexual partners of women with recurrent BV be treated with antibiotics at the same time. For same-sex partners or a first episode, the recommendation is to discuss the option with a healthcare provider. The recognition that partner treatment matters marks a meaningful shift in how the medical community views BV’s relationship to sexual health.
Douching and Scented Products
Douching is one of the most well-documented triggers. It directly alters vaginal pH and washes away the protective bacteria that keep harmful organisms in check. Some douching products contain antimicrobial agents that are specifically toxic to the beneficial bacteria the vagina depends on, while leaving the harmful anaerobes free to multiply. Women who douche to treat BV symptoms often make the problem worse, killing off healthy bacteria and fueling the cycle of overgrowth.
Shower gels, scented soaps, and liquid soaps used internally carry similar risks. These products aren’t formulated for vaginal use and can cause irritation, inflammation, and disruption of the normal flora, all of which increase susceptibility to infection.
Hormonal Changes
Estrogen plays a behind-the-scenes role in maintaining vaginal health. It stimulates the vaginal lining to produce glycogen, a type of sugar that protective bacteria feed on. When estrogen drops, glycogen production falls, and the bacteria that depend on it lose their food source. The result is a less acidic environment that’s more hospitable to the anaerobic bacteria associated with BV.
This is especially relevant during and after menopause. The sustained drop in estrogen leads to reduced growth and turnover of vaginal tissue, lower glycogen levels, and a measurable decline in protective bacteria. Postmenopausal women who aren’t on estrogen therapy tend to have vaginal environments colonized by a more diverse mix of anaerobic species, many of which are the same ones found in BV. Vaginal estrogen therapy has been shown to restore a healthier bacterial balance similar to that of premenopausal women.
Hormonal fluctuations during the menstrual cycle may also play a role. Menstrual blood temporarily raises vaginal pH, and some women notice BV symptoms recurring around their period.
Smoking
Cigarette smoking is linked to BV through a surprisingly direct biological pathway. Nicotine and its breakdown products accumulate in vaginal tissue. In women who already have low levels of protective bacteria, smoking is associated with significantly higher concentrations of compounds called biogenic amines, including cadaverine and putrescine. These substances raise vaginal pH, boost the ability of harmful bacteria to cause infection, and contribute to the characteristic fishy odor of BV.
The effect is essentially chemical: smoking changes the metabolic environment of the vagina in ways that favor the bacteria behind BV and weaken the conditions that protective bacteria need to thrive.
What BV Looks and Feels Like
Many women with BV have no symptoms at all. In the U.S. prevalence study, a substantial number of cases were found in women who weren’t experiencing any noticeable issues. When symptoms do appear, the hallmarks are a thin, milky white or gray discharge that coats the vaginal walls and a fishy odor that may be more noticeable after sex. The discharge tends to be homogeneous and smooth rather than clumpy.
Diagnosis typically involves checking for at least three of four signs: the characteristic thin discharge, a vaginal pH above 4.5, a fishy odor, and the presence of “clue cells” under a microscope (vaginal cells coated with bacteria, giving them a stippled appearance). BV is distinct from yeast infections, which usually cause thick, cottage cheese-like discharge and itching. If you’re unsure which you’re dealing with, testing is the only reliable way to tell.
Why BV Keeps Coming Back
Recurrence is the most frustrating aspect of BV. The biofilms formed by Gardnerella are a major reason. Even after a full course of antibiotics clears symptoms, biofilm bacteria can persist in the vagina and re-establish the infection. Neither of the two most commonly prescribed antibiotics has been shown to fully eradicate vaginal biofilms in laboratory studies.
Ongoing exposure to the same triggers, whether that’s an untreated partner, continued douching, or smoking, also keeps the cycle going. For women dealing with repeated episodes, addressing as many contributing factors as possible tends to be more effective than relying on antibiotics alone. That means stopping douching, using condoms, discussing partner treatment with a provider, and quitting smoking if applicable. Each of these removes one of the forces pushing the vaginal environment toward imbalance.