Bacterial vaginosis (BV) happens when the naturally protective bacteria in the vagina are overtaken by a mix of other organisms. It’s the most common vaginal infection in women of reproductive age, affecting roughly 20 to 30% worldwide, with rates reaching nearly 30% in the United States alone. Despite how common it is, the exact trigger isn’t fully understood. What researchers do know is that a specific shift in the vaginal ecosystem sets the stage, and several behaviors and factors make that shift more likely.
How the Vaginal Ecosystem Normally Works
A healthy vagina is dominated by bacteria from the Lactobacillus family, particularly four species: L. crispatus, L. gasseri, L. iners, and L. jensenii. These bacteria produce lactic acid, which keeps the vaginal pH between 3.5 and 4.5. That acidic environment is inhospitable to most harmful organisms. Estrogen plays a supporting role by promoting glycogen in vaginal cells, which Lactobacillus feeds on to produce that protective acid.
Think of it as a well-maintained garden. Lactobacillus acts like ground cover, taking up space and resources so weeds can’t establish themselves. When the ground cover thins out, the weeds move in.
What Changes During BV
BV is defined by a depletion of Lactobacillus and a surge in mixed anaerobic bacteria, organisms that thrive in low-oxygen, less acidic conditions. The usual suspects include Gardnerella, Prevotella, Mobiluncus, Atopobium, Mycoplasma, and Ureaplasma, among others. As these organisms multiply, lactic acid drops, vaginal pH climbs above 4.5, and the environment becomes increasingly favorable for even more anaerobes to colonize.
Gardnerella vaginalis plays a particularly important role. It adheres to vaginal cells more aggressively than other anaerobes and produces enzymes that break down the protective mucus lining of the vaginal wall. This damage makes it easier for other bacteria to attach and establish a foothold. Gardnerella also produces a toxin that punches holes in vaginal cells, directly damaging tissue.
The Biofilm Problem
One of the most important discoveries in BV research is that Gardnerella doesn’t just float around freely. It builds biofilms: structured bacterial communities encased in a protective matrix of sugars, proteins, and genetic material. These biofilms cling to the vaginal lining and act as a fortress. Compared to free-floating bacteria, biofilm-embedded Gardnerella is far more resistant to both lactic acid and hydrogen peroxide, the two main antimicrobial weapons a healthy vagina deploys.
This biofilm is also a major reason BV is so hard to get rid of permanently. Standard antibiotic treatment can clear the symptoms, but the biofilm often survives, sheltering bacteria from the medication. Recurrence rates exceed 50%, largely because the biofilm remains intact and allows the bacterial community to regrow once treatment ends.
Known Risk Factors
Researchers still can’t point to a single cause of BV. Instead, several factors are known to disrupt the vaginal balance and increase risk.
- New or multiple sexual partners. BV occurs most often in sexually active women, and having new partners is one of the strongest risk factors. The exact mechanism is unclear. One theory is that sexual activity introduces new bacteria or disrupts the existing community. BV can also spread between female sexual partners. Interestingly, treating a male partner does not appear to reduce a woman’s risk of developing or recurring BV.
- Douching. Rinsing the vagina with water or other solutions washes away Lactobacillus, raises vaginal pH, and creates an opening for harmful bacteria to colonize. Douching is also associated with higher rates of BV recurrence.
- Unprotected intercourse. Not using condoms is associated with increased BV risk, likely because semen is alkaline (pH around 7 to 8) and temporarily raises vaginal pH, giving anaerobes a window to gain ground.
- Smoking. Chronic smoking is linked to both initial BV and recurrent episodes. Chemicals from tobacco concentrate in cervical mucus and may impair the local immune response.
- IUD use. Intrauterine devices, particularly copper IUDs, may promote overgrowth of BV-associated bacteria. Copper IUDs can increase menstrual flow, and the iron in blood (heme) appears to stimulate Gardnerella growth specifically.
Symptoms and How It’s Diagnosed
Many women with BV have no symptoms at all. When symptoms do appear, the most recognizable is a thin, grayish-white discharge with a strong fishy odor, often more noticeable after sex. There’s typically no significant itching or burning, which helps distinguish BV from yeast infections.
Clinicians diagnose BV using a set of bedside findings called the Amsel criteria. A diagnosis requires at least three of the following four signs: the characteristic thin discharge, vaginal pH above 4.5, a fishy smell when a chemical solution is added to a sample (the “whiff test”), and the presence of “clue cells” under a microscope. Clue cells are vaginal cells so heavily coated in bacteria that their edges appear fuzzy or stippled. A lab-based scoring method using a Gram-stained sample can also confirm the diagnosis by quantifying the shift from Lactobacillus-dominant flora to mixed anaerobes.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. More than half of women who are successfully treated experience a return of symptoms. Several factors drive this pattern.
The biofilm is the primary culprit. Antibiotics reduce the bacterial load enough to relieve symptoms, but the biofilm on the vaginal lining acts as a reservoir. Once antibiotic levels drop, bacteria within the biofilm repopulate the vaginal canal. Antimicrobial resistance within the biofilm further complicates treatment, as the encased bacteria require much higher drug concentrations to kill than free-floating organisms.
Re-exposure from a regular sexual partner is another suspected contributor. While treating male partners hasn’t been shown to help in clinical trials, the possibility that BV-associated bacteria are exchanged during sex remains an active area of investigation. For female partners, transmission is more clearly established. Ongoing behavioral risk factors like continued douching or smoking also increase the chance of recurrence.
BV and Broader Health Risks
BV isn’t just an uncomfortable nuisance. The disrupted vaginal environment increases vulnerability to sexually transmitted infections, including HIV. Without the acidic, Lactobacillus-rich environment acting as a barrier, STI-causing organisms have an easier path to infect vaginal tissue. BV also appears to recur more frequently in women who already have HIV.
During pregnancy, BV roughly doubles the risk of preterm delivery. A large meta-analysis found that women with BV had about 1.8 times the odds of delivering prematurely compared to women without BV. This makes early detection and treatment particularly relevant for pregnant women.
Reducing Your Risk
Because the exact cause of BV remains unknown, prevention is about minimizing disruption to your vaginal ecosystem. Avoiding douching is one of the most effective steps, as it removes the very bacteria your body relies on for protection. Using condoms consistently with new or multiple partners also reduces risk. Quitting smoking helps both initial prevention and recurrence.
Probiotics have shown some promise. A meta-analysis of randomized controlled trials found that probiotics reduced BV recurrence by about 45% compared to placebo or standard antibiotics alone. However, no specific strain, dose, or delivery method (oral versus vaginal) stood out as clearly superior, and the overall evidence is still mixed. Probiotics are worth discussing with a healthcare provider, particularly for women dealing with repeated episodes, but they aren’t a reliable standalone solution.
The vagina is largely self-cleaning. Warm water on the outside is sufficient for hygiene. Scented soaps, sprays, and internal washes do more harm than good by disrupting the microbial balance that keeps BV and other infections at bay.