Bacterial vaginosis (BV) develops when the normal balance of bacteria in the vagina shifts, allowing certain harmful bacteria to outnumber the protective ones. It’s the most common vaginal infection in women of reproductive age, and despite how frequently it occurs, the exact trigger isn’t always clear. There’s no single way someone “catches” BV. Instead, several factors can tip the bacterial balance in the wrong direction.
What Happens Inside the Vagina
A healthy vagina is home to a community of bacteria dominated by Lactobacillus species, which produce lactic acid and keep the environment slightly acidic (below a pH of 4.5). This acidity acts as a natural defense, suppressing the growth of other microorganisms. BV occurs when Lactobacillus populations drop and anaerobic bacteria, organisms that thrive without oxygen, multiply and take over.
One bacterium in particular, Gardnerella vaginalis, plays a central role. It adheres to the vaginal walls more aggressively than other bacteria and forms a sticky, protective layer called a biofilm. Research published in The Journal of Infectious Diseases found biofilms on 90% of vaginal tissue samples from people with BV. This biofilm shields the harmful bacteria from the body’s own defenses, including the lactic acid and hydrogen peroxide that Lactobacillus normally produces. It also creates a foothold for other anaerobic bacteria to attach and grow, turning a minor imbalance into a full-blown infection.
Sexual Activity and New Partners
BV is not a sexually transmitted infection in the traditional sense. You can’t “catch” it the way you’d catch chlamydia or gonorrhea. But sexual activity is one of the strongest risk factors. Having a new sexual partner or multiple partners increases the likelihood of developing BV because it introduces unfamiliar bacteria into the vaginal environment. Unprotected sex further raises the risk, as semen is alkaline and can temporarily shift vaginal pH above the protective acidic range.
BV also occurs between female sexual partners. When two women share vaginal bacteria through skin-to-skin contact or shared toys, the same bacterial disruption can happen. Studies have found that female partners of women with BV are more likely to carry BV-associated bacteria themselves, suggesting the condition can be exchanged even without a male partner involved.
That said, people who have never been sexually active can still develop BV. Sex raises the risk, but it isn’t required.
Douching and Vaginal Products
Douching, the practice of flushing the inside of the vagina with water or a cleansing solution, is one of the most well-documented risk factors. It physically washes away protective Lactobacillus bacteria and disrupts the vaginal pH, creating exactly the conditions that allow anaerobic bacteria to flourish. The World Health Organization identifies vaginal cleansing and douching as clear contributors to BV risk.
Other intravaginal practices carry similar risks. Inserting herbs, scented products, or other substances into the vagina can irritate the vaginal lining and alter the bacterial community. Scented soaps, bubble baths, and even some laundry detergents used on underwear can have a milder version of the same effect. The vagina is largely self-cleaning, and most products marketed for vaginal hygiene do more harm than good.
What BV Feels and Looks Like
About half of people with BV don’t notice any symptoms at all. When symptoms do appear, the most recognizable one is a thin, grayish-white discharge with a distinctive fishy smell. The odor tends to be stronger after sex or during a period. The discharge has a milklike consistency and coats the vaginal walls evenly, which looks different from the clumpy discharge associated with yeast infections.
Some people also experience mild itching or burning, particularly during urination, though these symptoms are less common than with other vaginal infections. Doctors diagnose BV by checking for at least three clinical signs: the characteristic discharge, a vaginal pH above 4.5, the presence of certain altered cells under a microscope, and a fishy odor when the discharge is exposed to a chemical solution.
Why BV Keeps Coming Back
One of the most frustrating aspects of BV is its recurrence rate. Between 50% and 80% of people who are successfully treated with antibiotics experience a return of symptoms within 6 to 12 months. The biofilm that Gardnerella vaginalis creates is a major reason why. Standard antibiotics kill the free-floating bacteria effectively, but the biofilm acts like a shield, protecting bacteria embedded within it. Once the antibiotic course ends, the surviving bacteria in the biofilm can repopulate the vagina and restart the cycle.
Reinfection from a sexual partner is another possible driver. If your partner carries BV-associated bacteria, treatment may clear your symptoms temporarily while the same bacteria are reintroduced through sexual contact. This is an active area of clinical investigation, and some healthcare providers now consider treating partners as part of managing recurrent BV.
Reducing Your Risk
Because the exact cause varies from person to person, prevention focuses on protecting the vaginal bacterial balance. Using condoms consistently reduces risk by limiting the introduction of outside bacteria and the pH-disrupting effects of semen. Avoiding douching is one of the simplest and most impactful steps you can take. Sticking with plain water for external genital washing, rather than scented soaps or specialized products, helps preserve the vagina’s natural environment.
Probiotics are a growing area of interest. A clinical trial of 228 women found that those who used a vaginal probiotic containing Lactobacillus crispatus after standard antibiotic treatment had a 30% recurrence rate at 12 weeks, compared with 45% in the group that didn’t use the probiotic. That’s a meaningful difference, though it also shows probiotics aren’t a complete solution on their own.
BV During Pregnancy
BV during pregnancy is associated with a higher chance of preterm delivery, meaning birth before 37 weeks. Preterm delivery carries serious risks for the baby, including breathing problems, bleeding in the brain, and in severe cases, death. However, the relationship is complicated. The U.S. Preventive Services Task Force notes that while BV and preterm delivery are linked, it remains unclear whether BV directly causes early labor. If you’re pregnant and notice symptoms of BV, bringing it up with your provider is worthwhile so they can weigh the risks and benefits of treatment for your specific situation.