Bacterial vaginosis (BV) happens when the natural balance of bacteria in the vagina shifts. Protective bacteria called Lactobacillus, which normally dominate the vaginal environment, decline and get replaced by an overgrowth of anaerobic bacteria. It’s the most common gynecological condition in women of reproductive age, and despite how frequently it occurs, the exact trigger isn’t always easy to pin down.
The Bacterial Shift Behind BV
A healthy vagina is home to large populations of Lactobacillus species. These bacteria produce hydrogen peroxide and lactic acid, which keep the vaginal pH acidic (typically below 4.5) and make the environment inhospitable to harmful microbes. When Lactobacillus numbers drop, that acidic protection weakens, and anaerobic bacteria seize the opportunity to multiply.
The bacteria that take over include Gardnerella vaginalis, Prevotella species, Mobiluncus species, and Atopobium vaginae, among others. Rather than a single “infection” caused by one germ, BV is a community-wide disruption. Multiple species work together, forming what’s known as a polymicrobial biofilm on the vaginal lining. This biofilm is essentially a protective shield: the bacteria embed themselves in a matrix (more than 50% protein) that helps them resist both the body’s natural defenses and antibiotic treatment. That biofilm is a major reason BV tends to come back.
Known Risk Factors
Several behaviors and circumstances increase the likelihood of this bacterial imbalance:
- Douching. Rinsing the vagina with water or cleansing solutions washes away Lactobacillus and disrupts the natural pH. This is one of the most well-established risk factors.
- New or multiple sexual partners. Sexual activity can introduce unfamiliar bacteria into the vaginal environment, shifting the microbial balance. The more partners, the greater the exposure to different bacterial communities.
- Not using condoms. Semen is alkaline, which temporarily raises vaginal pH. Consistent condom use appears to help maintain the acidic environment that Lactobacillus needs.
BV can also occur in people who have never been sexually active, which makes it clear that sex isn’t the sole cause. Anything that disturbs the vaginal ecosystem, from hormonal changes to the use of scented soaps or body washes near the vulva, can potentially tip the balance.
Is BV a Sexually Transmitted Infection?
This is one of the most common questions, and the answer is nuanced. BV is not classified as a sexually transmitted infection. You can’t “catch” it the way you’d catch chlamydia or gonorrhea. However, sexual activity is closely linked to BV risk. New partners, unprotected sex, and sex between women (where vaginal bacteria can be shared directly) all increase the chance of developing it. The current understanding is that sex can be a trigger for the microbial disruption, but the condition itself is a shift in your own vaginal ecosystem rather than an invasion by a single transmitted pathogen.
How BV Is Identified
Doctors typically use a set of clinical signs to diagnose BV. The most common approach looks for at least three of four indicators: a thin, grayish-white vaginal discharge that looks uniform rather than clumpy; a vaginal pH above 4.5; a fishy odor when a chemical solution is added to a sample of the discharge (called a whiff test); and the presence of “clue cells,” which are vaginal lining cells coated in bacteria, visible under a microscope. If clue cells make up 20% or more of the visible cells, that marker is considered positive.
Many people with BV have no symptoms at all. When symptoms do appear, the hallmark is a noticeable fishy smell, especially after sex, along with thin discharge. Itching and irritation are less common with BV than with yeast infections, which is one way to tell the two apart before seeing a provider.
Why BV Keeps Coming Back
Recurrence is frustratingly common. Even after successful antibiotic treatment, a significant number of people develop BV again within months. The biofilm that Gardnerella and its companion bacteria create on vaginal cells is a big part of the problem. Antibiotics can kill the free-floating bacteria, but the biofilm acts as a fortress, sheltering bacteria from the medication. Once treatment stops, the surviving bacteria within the biofilm repopulate the vagina and the cycle restarts.
A clinical trial published in the New England Journal of Medicine tested whether replenishing Lactobacillus after antibiotic treatment could break this cycle. After standard antibiotic therapy, 152 women used a vaginal probiotic containing Lactobacillus crispatus twice per week for 11 weeks. Only 30% of those women had BV come back by week 12, compared with 45% in the group that received a placebo. That’s a meaningful reduction, though it shows that even with probiotic support, recurrence remains possible for some.
BV During Pregnancy
BV carries additional concern during pregnancy. People with BV while pregnant may face a higher risk of preterm delivery, meaning birth before 37 weeks. Preterm birth is associated with serious complications for newborns, including breathing problems, bleeding in the brain, and in severe cases, death. The relationship between BV and preterm delivery is well-documented statistically, though researchers haven’t yet confirmed that BV directly causes early labor rather than being associated with other contributing factors.
Reducing Your Risk
Because BV stems from an ecosystem disruption rather than a single invading germ, prevention focuses on protecting the vaginal environment. Avoiding douching is the single most impactful step. The vagina is self-cleaning, and introducing water, vinegar, or commercial douching products does more harm than good. Using condoms consistently, especially with new partners, helps buffer the vaginal pH from the alkaline effect of semen and limits exposure to unfamiliar bacteria.
Choosing unscented soaps for the external genital area, wearing breathable cotton underwear, and avoiding prolonged time in wet swimwear are all small habits that support a stable vaginal environment. None of these are guarantees, but they reduce the odds of tipping the balance away from Lactobacillus dominance. If you experience recurrent BV, talking to a healthcare provider about probiotic strategies or extended treatment options can help you find an approach tailored to your pattern.