Bacterial vaginosis (BV) happens when the balance of bacteria in the vagina shifts. Normally, beneficial bacteria called lactobacilli keep the vaginal environment acidic, which prevents other organisms from taking over. When those protective bacteria decline, other types of bacteria multiply rapidly, and that overgrowth is BV. It’s the most common vaginal condition in women ages 15 to 44, and it’s not caused by a single event or germ. It results from a disruption in an entire ecosystem.
What Happens Inside the Vagina
A healthy vagina maintains a pH between 3.8 and 4.5, which is moderately acidic. Lactobacilli are responsible for that acidity. They break down glycogen (a sugar stored in vaginal cells) into lactic acid, and that acid creates an environment where harmful bacteria struggle to survive. Lactobacilli also produce hydrogen peroxide and other antimicrobial compounds, and they physically crowd out other organisms by occupying space on the vaginal lining.
When lactobacilli decline for any reason, the pH rises above 4.5. That higher pH favors a different set of bacteria, particularly a species called Gardnerella vaginalis. Gardnerella is actually present in small amounts in many healthy vaginas, but once conditions shift in its favor, it multiplies aggressively and forms a sticky biofilm on the vaginal wall. This biofilm is far more resistant to the body’s natural defenses than free-floating bacteria, which is one reason BV can be stubborn to treat and quick to return.
Once Gardnerella establishes itself, it produces enzymes that break down the protective mucus layer of the vaginal lining. That breakdown makes it easier for other anaerobic bacteria (organisms that thrive without oxygen) to attach and grow. The result is a dense, multi-species bacterial community that replaces the lactobacillus-dominated environment. This is what produces the hallmark symptoms of BV: thin grayish-white discharge and a fishy odor.
The Most Common Triggers
BV isn’t caused by poor hygiene. In fact, one of the most well-documented risk factors is too much cleaning. Douching, whether with water, saline, vinegar, or antiseptic solutions, disrupts the vaginal microbiome rapidly. Studies show that douching can reduce the number and diversity of vaginal bacteria within 10 minutes, and using antiseptic agents like povidone-iodine can actually trigger the overgrowth of the exact organisms that cause BV by suppressing lactobacilli. Women who douche have roughly twice the risk of developing a vaginal infection compared to those who don’t.
Sexual activity is another major factor, though BV is not a sexually transmitted infection in the traditional sense. New sexual partners, multiple partners, and unprotected sex all increase risk. Semen is alkaline, and exposure to it temporarily raises vaginal pH above 4.5, which is the threshold where lactobacilli begin to struggle and BV-associated bacteria thrive. This doesn’t mean every sexual encounter triggers BV, but repeated exposure to semen without condom use can create conditions that favor bacterial imbalance over time. BV also occurs more frequently in women who have sex with women, likely because vaginal bacteria can be shared between partners.
Hormonal changes play a role too. Estrogen drives the production of glycogen in vaginal cells, and glycogen is what lactobacilli feed on. When estrogen drops (during menstruation, after menopause, or with certain hormonal contraceptives), the food supply for protective bacteria shrinks, leaving the door open for other organisms.
Other Factors That Raise Your Risk
- Antibiotics: Taking antibiotics for any infection can kill off lactobacilli along with the targeted bacteria, temporarily leaving the vagina vulnerable.
- Scented products: Fragranced soaps, bubble baths, and vaginal deodorants can irritate the vaginal lining and alter its chemistry.
- Smoking: Tobacco use is consistently linked to higher BV rates, possibly because it reduces lactobacillus populations.
- IUDs: Some studies associate intrauterine devices with a modestly higher risk of BV, though the evidence is mixed.
What BV Feels Like
About half of women with BV have no symptoms at all. When symptoms do appear, the most noticeable one is a thin, milky or grayish discharge with a distinctly fishy smell, often stronger after sex or during a period. Unlike a yeast infection, BV rarely causes significant itching or thick, clumpy discharge. There’s usually no redness or swelling either.
A clinician diagnoses BV by checking for at least three of four signs: the characteristic thin discharge, a vaginal pH above 4.5, a fishy odor when a chemical solution is added to a sample, and the presence of “clue cells” under a microscope (vaginal cells coated with bacteria). These are known as the Amsel criteria. The condition is treatable with prescription antibiotics, but timing and completion of the course matter because of how persistent Gardnerella biofilms can be.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. Between 50% and 80% of women who complete antibiotic treatment will have BV again within 6 to 12 months. The Gardnerella biofilm is a big part of the problem. Antibiotics can kill the bacteria floating freely in vaginal fluid, but the biofilm acts as a shield, protecting organisms embedded in it. Once the antibiotic course ends, bacteria surviving in the biofilm can repopulate quickly.
Reinfection from a sexual partner is another possible contributor. If a partner carries BV-associated bacteria (which can colonize penile skin and the urethra), those organisms may be reintroduced with each sexual encounter. Using condoms consistently can reduce this cycle of reexposure, and some clinicians now consider treating partners as part of a recurrence-prevention strategy.
Lowering Your Risk
The vagina is largely self-cleaning, and the single most effective thing you can do is stop disrupting its natural chemistry. That means no douching, no internal washing with soap, and avoiding scented products in the genital area. Warm water on the external vulva is sufficient for hygiene.
Consistent condom use reduces exposure to semen’s alkaline pH and limits the exchange of bacteria between partners. If you notice that BV tends to follow your period (when blood temporarily raises vaginal pH), that pattern is worth mentioning to your healthcare provider, as some people benefit from targeted strategies around menstruation.
Probiotics containing lactobacillus strains are widely marketed for vaginal health, but the evidence supporting them for BV prevention is still inconsistent. Some small studies show benefit, particularly when used alongside antibiotics, but no specific product has strong enough data to be a standard recommendation yet. Maintaining overall health, avoiding unnecessary antibiotics when possible, and not smoking are more reliably supported ways to keep your vaginal microbiome stable.