Bacterial vaginosis (BV) is caused by a shift in the vaginal microbiome: the protective bacteria that normally dominate the vagina decline, and a mix of other bacteria overgrow in their place. It’s the most common vaginal condition in women of reproductive age, and despite decades of research, the exact trigger for this bacterial shift isn’t fully understood. What is clear is that several specific factors make the imbalance more likely to happen.
How the Vaginal Microbiome Normally Works
A healthy vagina is home to a low-diversity community of bacteria dominated by species of Lactobacillus. These bacteria produce lactic acid, keeping the vaginal pH below 4.5, which is acidic enough to suppress the growth of harmful organisms. This acidic environment acts as the first line of defense against invading pathogens.
There are several types of healthy vaginal microbiome profiles, but they all share one feature: Lactobacillus species like L. crispatus, L. gasseri, or L. iners make up the overwhelming majority of bacteria present. When something disrupts that dominance, the door opens for a diverse mix of anaerobic bacteria (ones that thrive without oxygen) to multiply. That overgrowth is BV.
What Triggers the Bacterial Shift
No single cause has been identified. Instead, BV appears to result from a combination of factors that disturb the vaginal environment. The CDC lists several well-established associations:
- A new sexual partner or multiple partners. Sexual activity is the strongest and most consistent risk factor. Women who have never been sexually active are rarely affected. BV is linked to new male partners, female partners, and relationships involving more than one person. However, BV is not classified as a sexually transmitted infection because it can occur in women without these risk factors, and whether a single transmissible pathogen is responsible remains unknown.
- Lack of condom use. Unprotected sex is associated with higher BV rates, possibly because semen is alkaline and temporarily raises vaginal pH, or because partner bacteria are introduced directly.
- Douching. Vaginal douching physically washes out Lactobacillus bacteria. Even vinegar-based douches don’t replicate the protection of natural lactic acid. As one microbiome researcher at Fred Hutch put it, “all acid is not created equal, and acetic acid cannot substitute for lactic acid.” The practice disrupts the very bacteria responsible for keeping the vaginal environment stable.
Other associations include HSV-2 infection (genital herpes) and general disruption of the vaginal environment through products like scented soaps, bubble baths, or intrauterine devices, though these links are less firmly established than the ones above.
Is BV Sexually Transmitted?
This is one of the most debated questions in the field, and the honest answer is: we don’t know for certain. BV behaves like a sexually associated condition rather than a classic STI. The evidence pointing toward sexual transmission is strong. New partners and unprotected sex are consistent risk factors, and the condition is rare in people who haven’t had sex. But BV doesn’t fit neatly into the STI category either. No single organism has been identified as “the” pathogen, and treating male sexual partners with antibiotics hasn’t reliably prevented recurrence in studies.
The current medical consensus treats BV as sexually associated but not sexually transmitted, a distinction that matters because it shapes treatment decisions and partner management.
What BV Looks and Feels Like
Many women with BV have no symptoms at all. When symptoms do appear, the most recognizable is a thin, grayish-white vaginal discharge with a fishy odor, particularly noticeable after sex. The vaginal pH rises above 4.5, reflecting the loss of acid-producing Lactobacillus. Unlike yeast infections, BV typically doesn’t cause significant itching or thick, chunky discharge. The odor is often what prompts women to seek care.
Clinicians diagnose BV by checking for at least three of four markers: the characteristic thin discharge, a vaginal pH above 4.5, a fishy smell when a chemical solution is applied to a sample, and the presence of “clue cells” (vaginal cells coated with bacteria) under a microscope.
Why BV Keeps Coming Back
Recurrence is the defining frustration of BV. Within 6 to 12 months after finishing antibiotic treatment, 50% to 80% of women experience the condition again. Antibiotics effectively kill the overgrown bacteria, but they don’t always allow Lactobacillus to recolonize and re-establish dominance. If the protective bacteria don’t bounce back, the cycle repeats.
Several factors feed recurrence. Ongoing exposure to the same risk factors (a partner whose microbiome reintroduces disruptive bacteria, continued douching) can re-trigger the imbalance. Some women also appear to have a vaginal microbiome that is inherently less stable, making them more vulnerable regardless of behavior changes.
Reducing the Risk of BV
Because the exact cause is unclear, prevention focuses on protecting the Lactobacillus population. Avoiding douching is the single most actionable step, since it directly removes the bacteria your body needs. Consistent condom use with new or multiple partners reduces exposure to microbiome-disrupting organisms. Avoiding scented products in or around the vagina helps preserve the natural chemical 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 (applied twice weekly for 11 weeks after standard antibiotic treatment) had a 30% recurrence rate at 12 weeks, compared with 45% in the placebo group. That’s a meaningful reduction, though not a cure. Vaginal probiotics are showing more promise than oral ones for this purpose, likely because they deliver bacteria directly where they’re needed.
For women dealing with repeated episodes, the pattern itself is worth discussing with a healthcare provider. Extended or suppressive antibiotic regimens, combined with probiotic strategies, represent the current approach to breaking the recurrence cycle.