Bacterial vaginosis (BV) is caused by a shift in the vaginal microbiome: the protective bacteria that normally dominate the vagina decline sharply, and a mix of other bacteria overgrow to take their place. It’s not a single infection caught from one source. It’s a disruption of an entire ecosystem, and it’s the most common vaginal condition in women ages 15 to 44.
How the Vaginal Microbiome Normally Works
A healthy vagina is dominated by bacteria called Lactobacillus. These bacteria produce lactic acid and other antimicrobial compounds that keep the vaginal environment acidic, typically between a pH of 3.8 and 4.5. That acidity acts like a chemical barrier. It suppresses the growth of most harmful organisms and keeps the overall bacterial community stable.
When this system is working, the vagina is largely self-regulating. The Lactobacillus population crowds out potential troublemakers, and the low pH makes it difficult for them to gain a foothold.
What Changes When BV Develops
BV begins when the Lactobacillus population drops and anaerobic bacteria (species that thrive in low-oxygen environments) multiply to fill the gap. The vaginal pH rises above 4.5, which further favors the newcomers and makes it harder for Lactobacillus to recover. It becomes a self-reinforcing cycle: fewer protective bacteria means higher pH, which means even fewer protective bacteria.
The most commonly identified organism in BV is Gardnerella vaginalis. It plays a specific architectural role: it attaches strongly to the cells lining the vaginal wall and forms a biofilm, a thin, sticky layer of bacteria embedded in a protective matrix. That biofilm then acts as scaffolding. Other anaerobic species attach to it and begin to thrive in the low-oxygen pockets the biofilm creates.
This layered community is what makes BV stubborn. The biofilm shields the bacteria inside it from both the immune system and antibiotics. Some of the secondary species actually ramp up the damage. When certain bacteria partner with Gardnerella in the biofilm, they trigger increased production of enzymes that break down the protective mucus layer on the vaginal wall, making the tissue more vulnerable. This multi-species cooperation is a key reason why BV has such high recurrence rates: 50% to 80% of women who finish antibiotic treatment experience BV again within 6 to 12 months.
Known Triggers and Risk Factors
No single behavior definitively “causes” BV, but several factors reliably increase the risk by disrupting the vaginal microbiome.
Douching is one of the strongest and most preventable risk factors. It washes away Lactobacillus and disrupts the natural acidity of the vagina. Women who douche weekly are five times more likely to develop BV than women who don’t douche at all. Despite marketing that frames douching as hygienic, it works against the vagina’s built-in self-cleaning system.
Sexual activity is consistently linked to BV, though the relationship is complicated. BV is not classified as a sexually transmitted infection because it occurs in people who have never had sex. But new sexual partners, multiple partners, and unprotected sex all raise the risk. One likely mechanism: semen has a pH around 7.2 to 8.0, which is significantly more alkaline than the vaginal environment. Repeated exposure can temporarily raise vaginal pH enough to give anaerobic bacteria an opening. Sexual contact can also introduce new bacteria directly.
Other factors that contribute include antibiotic use (which can kill Lactobacillus along with whatever infection is being treated), hormonal changes, smoking, and the use of scented soaps or products inside or near the vagina.
How BV Is Recognized
BV often produces a thin, grayish-white discharge with a distinct fishy smell, particularly noticeable after sex. But roughly half of women with BV have no obvious symptoms at all, which means it can persist undetected.
Clinicians typically diagnose BV using a combination of signs: the characteristic thin discharge, a vaginal pH above 4.5, a fishy odor when a chemical solution is applied to a sample of the discharge, and the presence of “clue cells” under a microscope. Clue cells are normal vaginal cells that have become so coated with bacteria they look stippled or fuzzy around the edges. Three of these four signs confirm the diagnosis.
Why It Matters Beyond Discomfort
BV is easy to dismiss as a nuisance, but untreated BV carries real health consequences. Women with symptomatic BV have roughly double the risk of developing pelvic inflammatory disease (PID), an infection of the uterus, fallopian tubes, or ovaries that can cause chronic pain and fertility problems. BV also increases susceptibility to sexually transmitted infections, including chlamydia, gonorrhea, and HIV, likely because the disrupted mucosal barrier and higher pH create a less hostile environment for incoming pathogens.
During pregnancy, BV is associated with preterm birth and low birth weight. The inflammatory response triggered by the overgrowth of anaerobic bacteria can affect the membranes surrounding the fetus, potentially leading to early labor.
Why BV Keeps Coming Back
The recurrence problem is one of the most frustrating aspects of BV. Standard antibiotic treatment clears the symptoms effectively in most cases, but the biofilm on the vaginal wall is difficult to fully eradicate. Even a small remnant can serve as the foundation for regrowth. The surviving bacteria, protected within the biofilm, begin repopulating as soon as the antibiotic course ends.
Reinfection from a sexual partner may also play a role. Research has found BV-associated bacteria, including Gardnerella, in the genital tracts of male partners. If only one partner is treated, the bacteria can be reintroduced. This is an active area of clinical investigation, with some studies now testing whether treating both partners simultaneously reduces recurrence.
Lifestyle adjustments that support Lactobacillus recovery, like avoiding douching, minimizing exposure to scented products near the vagina, and using condoms with new partners, can help reduce the odds of recurrence. Some clinicians also recommend extended or suppressive antibiotic courses for women with frequent episodes, though this approach involves tradeoffs with antibiotic resistance and side effects.