Bacterial vaginosis (BV) happens when the balance of bacteria inside the vagina shifts. Protective bacteria decline sharply, and a mix of other microbes multiply to 100 to 1,000 times their normal concentration. It’s the most common vaginal condition in women of reproductive age, and understanding what drives that bacterial shift helps explain why it develops, why it recurs, and what you can do to lower your risk.
The Bacterial Shift Behind BV
A healthy vagina is dominated by Lactobacillus bacteria. These organisms produce lactic acid, which keeps the vaginal environment slightly acidic, typically between a pH of 3.8 and 4.5. That acidity acts like a natural defense system, suppressing the growth of harmful microbes.
In BV, Lactobacillus numbers drop dramatically. The bacteria that replace them are a diverse group of anaerobes, organisms that thrive in low-oxygen environments. The most well-known is Gardnerella, but others including Prevotella, Atopobium, Mobiluncus, and Sneathia are commonly involved. As these bacteria multiply, they raise the vaginal pH above 4.5. The environment becomes less acidic, which allows even more of these organisms to flourish. It’s a self-reinforcing cycle: once the shift starts, it tends to accelerate.
This overgrowth also produces chemical byproducts, including trimethylamine, which is responsible for the fishy odor many people associate with BV. The discharge itself changes too, becoming thin, grayish-white, and milklike in consistency.
Why the Infection Is Hard to Clear
One reason BV is so persistent has to do with biofilms. Gardnerella and other BV-associated bacteria can form a sticky, protective layer that adheres to the vaginal wall. This biofilm shields the bacteria from the body’s natural defenses and from treatment. Lab studies show that biofilms made primarily of Gardnerella can survive exposure to hydrogen peroxide, lactic acid, and even high concentrations of antibiotics. When researchers dissolved the biofilm with enzymes in the lab, the bacteria became vulnerable again, but inside the body, the biofilm often remains intact after a course of treatment.
This helps explain BV’s notoriously high recurrence rate. Standard treatment clears the infection in 80% to 90% of cases initially, but up to 60% of people experience a recurrence within 12 months. The biofilm essentially acts as a reservoir, allowing a small population of bacteria to survive and repopulate once conditions are right.
Known Risk Factors
BV is not classified as a sexually transmitted infection, but sexual activity is one of the strongest risk factors. Having new or multiple sexual partners increases the likelihood of developing BV, likely because exposure to different bacteria can disrupt the existing vaginal ecosystem. Not using condoms also raises the risk, since semen is alkaline and temporarily raises vaginal pH. Studies consistently show higher rates of BV among sexually active women, and the condition can also pass between female sexual partners.
Douching is another well-established trigger. Rinsing the vagina with water, vinegar, or commercial products washes away Lactobacillus and other protective bacteria. The vagina is self-cleaning, and douching disrupts that process directly. The CDC lists douching as a primary risk factor alongside sexual activity.
Smoking and Vaginal Health
Cigarette smoking has a surprisingly direct connection to BV. Nicotine doesn’t just circulate in the blood; it’s also found in cervical mucus. Once there, it disrupts the regulation of the vaginal microbiome and promotes uncontrolled growth of potentially harmful organisms. Smoking also weakens both the immune system overall and the local immune defenses in the genital tract. The chemical components of tobacco smoke increase the permeability of vaginal tissue, triggering an influx of inflammatory cells that damage the protective connective tissue. At the same time, smoking suppresses the growth of protective Lactobacillus, destroying the mucus barrier that normally prevents harmful bacteria from attaching to vaginal cells. Nicotine can even enhance the ability of certain bacteria to form biofilms, making infections harder to resolve.
What BV Feels Like
Many people with BV have no symptoms at all. When symptoms do appear, the most common ones are a thin, grayish-white discharge that coats the vaginal walls and a noticeable fishy smell, often stronger after sex. Some people experience mild itching or burning, though intense itching is more typical of a yeast infection than BV. The discharge is usually not thick or clumpy.
Clinicians diagnose BV by checking for at least three of four signs: the characteristic thin discharge, a vaginal pH above 4.5, a fishy odor (sometimes only detectable after applying a chemical solution), and the presence of “clue cells” under a microscope, which are vaginal cells visibly coated with bacteria.
BV During Pregnancy
BV during pregnancy has been linked to a higher chance of preterm delivery, though the exact relationship is still being studied. The U.S. Preventive Services Task Force notes that people with BV during pregnancy may be more likely to deliver early, but whether BV directly causes preterm labor or simply occurs alongside other contributing factors remains unclear. If you’re pregnant and notice symptoms of BV, bringing it up with your provider is worthwhile since treatment during pregnancy is straightforward.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. Even with aggressive treatment approaches that combine antibiotics with follow-up maintenance therapy, recurrence rates hover around 30% at six months. Without maintenance therapy, rates climb to 60% within a year. The biofilm problem is a major factor, but lifestyle triggers matter too. If the original cause of the bacterial shift is still present, whether that’s douching, smoking, or a change in sexual partners, the conditions for BV to return haven’t changed.
Some people also have a vaginal microbiome that is naturally less dominated by the most protective Lactobacillus species. Not all Lactobacillus strains offer the same defense. One species commonly found in the vagina, Lactobacillus iners, cannot produce d-lactic acid, which plays a more important role in maintaining acidity than the l-lactic acid form. People whose microbiome is dominated by this less protective species may be more susceptible to BV in the first place and more prone to recurrence after treatment.
Reducing Your Risk
You can’t control every factor that influences your vaginal microbiome, but the most impactful steps are also the most straightforward. Avoiding douching removes one of the biggest disruptors of vaginal flora. Using condoms reduces exposure to bacteria and alkaline semen that can shift vaginal pH. Quitting smoking eliminates a source of direct chemical damage to the vaginal environment. Wearing breathable cotton underwear and avoiding scented soaps or sprays near the genital area may also help, though the evidence for these is less robust than for the first three.
BV is not caused by poor hygiene. In fact, over-cleaning is more likely to trigger it than under-cleaning. The vagina maintains its own ecosystem, and the most effective approach is to avoid disrupting it.