Bacterial vaginosis (BV) happens when the protective bacteria in the vagina are overtaken by other types of bacteria that normally exist there in small numbers. It affects roughly 23% to 29% of women of reproductive age worldwide, making it the most common vaginal condition in that age group. BV isn’t caused by a single germ the way a cold or strep throat is. Instead, it’s a shift in the entire bacterial ecosystem inside the vagina.
What Actually Happens Inside the Vagina
A healthy vagina is dominated by bacteria called Lactobacillus. These bacteria produce lactic acid, which keeps the vaginal pH low (acidic) and creates an environment hostile to most harmful microbes. BV develops when something disrupts this balance, Lactobacillus numbers plummet, and a mix of anaerobic bacteria rapidly fills the gap. The key species involved include Gardnerella vaginalis, Atopobium vaginae, Prevotella, Mobiluncus, and several others.
The process appears to follow a cascade. Certain strains of Gardnerella vaginalis attach to the vaginal lining and begin forming a sticky biofilm. As these bacteria break down proteins in the vaginal environment, they create conditions that help other species, particularly Prevotella, thrive. Prevotella produces ammonia, which further raises the pH and fuels more Gardnerella growth. Together, these bacteria degrade the protective mucus layer on the vaginal walls, allowing even more BV-associated species to join the biofilm. The result is a self-reinforcing cycle of bacterial overgrowth.
The exact trigger that kicks off this cascade remains unknown. Researchers understand the mechanics of how the bacterial shift unfolds, but not always what sets it in motion for any individual person.
Sexual Activity and BV Risk
Sexual activity is one of the strongest risk factors for developing BV, though BV is not classified as a sexually transmitted infection. People who have never had sex rarely get it, and the risk increases with new or multiple sexual partners.
One clear mechanism: semen is alkaline, with a pH around 7.2 to 8.0, compared to a healthy vaginal pH below 4.5. Exposure to semen temporarily raises vaginal pH, which can create a friendlier environment for anaerobic bacteria to multiply. This doesn’t mean every sexual encounter causes BV, but repeated shifts in pH can give opportunistic bacteria an opening. BV also occurs at higher rates among women who have sex with women, which suggests that vaginal bacteria can be shared between partners through direct contact, not just through semen exposure.
Douching and Hygiene Products
Douching is one of the most well-documented risk factors for BV. Research shows that water douches temporarily wash out Lactobacillus, and vinegar-based solutions don’t fix the problem because acetic acid cannot substitute for the lactic acid that Lactobacillus naturally produces. Nearly one in five U.S. women of reproductive age douche within a given year, despite strong evidence linking the practice to BV, pelvic inflammatory disease, and preterm birth in pregnant women.
Scented soaps, bubble baths, vaginal deodorants, and perfumed washes can also disrupt vaginal chemistry. The vagina is self-cleaning, and introducing products that alter its pH or irritate its lining creates the same kind of opening for anaerobic bacteria that douching does.
Antibiotics as an Overlooked Trigger
Broad-spectrum antibiotics taken for unrelated infections (a UTI, sinus infection, or dental procedure, for example) can kill off Lactobacillus along with the targeted bacteria. Since Lactobacillus is what keeps the vaginal environment acidic and stable, losing it to antibiotic treatment can tip the balance toward BV-associated species. This is one reason some people develop BV seemingly “out of nowhere” after an illness.
Smoking and Other Lifestyle Factors
Cigarette smoking has a dose-dependent relationship with BV risk, meaning the more you smoke, the higher the risk. The exact biological pathway isn’t fully understood, but smoking alters the vaginal metabolic environment in ways that appear to reduce Lactobacillus populations. Stress, hormonal changes (including those during menstruation), and the use of certain intrauterine devices have also been associated with shifts in vaginal flora.
What BV Feels Like
About half of people 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, particularly after sex. The discharge tends to have a milklike consistency and coats the vaginal walls evenly. Some people notice mild itching or burning during urination, but BV typically doesn’t cause the intense itching or thick, clumpy discharge associated with yeast infections.
The fishy smell comes from compounds called amines, which are produced by the anaerobic bacteria involved in BV. These amines become more volatile at higher pH levels, which is why the odor often intensifies after unprotected sex (when semen raises vaginal pH) or during menstruation.
Why BV Keeps Coming Back
One of the most frustrating aspects of BV is its recurrence rate. Between 50% and 80% of women experience a return of BV within 6 to 12 months of completing antibiotic treatment. The standard treatment, metronidazole, effectively kills off anaerobic bacteria, but the vaginal microbiome often doesn’t fully bounce back to its pre-BV state. Lactobacillus species may increase during the first few days of treatment, but these gains are frequently temporary.
A major reason for this is the biofilm that Gardnerella vaginalis and related species build on the vaginal lining. These biofilms can persist even through a full course of antibiotics because the drug doesn’t penetrate the biofilm effectively. The bacteria sheltering inside the biofilm survive treatment and rapidly repopulate once the antibiotic course ends. After treatment, the vaginal environment is often dominated by Lactobacillus iners, a species that provides less protection than other Lactobacillus types, leaving the door open for recurrence.
Reducing Your Risk
Because the exact trigger for BV varies from person to person, prevention isn’t always straightforward. But several factors are within your control:
- Skip douching entirely. The evidence against it is overwhelming, and the vagina maintains itself without it.
- Avoid scented products in or around the vaginal area, including perfumed soaps, sprays, and bath additives.
- Use condoms. Barrier methods reduce semen exposure and limit the exchange of bacteria between partners.
- Quit or reduce smoking. The dose-dependent relationship means even cutting back may lower risk.
- Be aware of antibiotic effects. If you develop vaginal symptoms during or after a course of antibiotics for another condition, that timing is not coincidental.
BV is not caused by poor hygiene. In fact, overcleaning the vaginal area is more likely to cause BV than undercleaning. The condition reflects a disruption in a delicate microbial balance, and in many cases, the trigger is something the body encounters routinely, like a change in sexual partners, hormonal fluctuations, or an unrelated medication.