Bacterial vaginosis (BV) develops when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment, producing lactic acid that keeps the pH between 3.5 and 4.5. When those protective bacteria decline and are replaced by a mix of anaerobic bacteria, the result is BV. It affects roughly 23 to 29 percent of women of reproductive age worldwide, making it the most common vaginal condition in that group.
BV isn’t caused by a single germ the way strep throat or a urinary tract infection might be. It’s a disruption of an entire ecosystem, and several things can trigger that disruption.
What Happens Inside the Vagina
A healthy vaginal environment works like a self-regulating defense system. Lactobacilli coat the vaginal walls in a thin layer and produce lactic acid along with other antimicrobial compounds. That acidic environment prevents harmful organisms from gaining a foothold.
When something disrupts this balance, anaerobic bacteria begin to multiply. One of the key players is a bacterium called Gardnerella vaginalis, which has an unusual ability to stick firmly to vaginal cells and build a structured colony called a biofilm. This biofilm acts as scaffolding that other BV-associated bacteria latch onto, creating a complex community that’s difficult for the immune system to clear. The biofilm also helps these bacteria tolerate antibiotics and resist the body’s defenses, which is a major reason BV tends to come back after treatment.
As these bacteria take over, they raise the vaginal pH above 4.5 and produce compounds that give off a characteristic fishy odor. Vaginal cells become coated with clinging bacteria (clinicians call these “clue cells”), and discharge becomes thin and milky.
Sexual Activity and New Partners
Sex is the strongest and most consistent risk factor for developing BV, though the relationship is complicated. BV occurs more frequently in people who have new or multiple sexual partners, and it’s more common when condoms aren’t used. Having a partner who also has BV increases your risk as well.
BV-associated bacteria can be exchanged through several types of sexual contact: shared sex toys, oral-genital contact, and manual stimulation with fingers. This means BV affects people across all types of sexual relationships, including between female partners. Many specialists now consider BV a sexually transmitted infection, although not every recurrence requires sexual exposure. The vaginal ecosystem can sometimes tip on its own.
If you’re looking to lower your risk, using latex condoms or dental dams during sex and limiting the number of sexual partners both reduce the odds of developing BV.
Douching and Hygiene Practices
Douching is one of the clearest non-sexual risk factors. Flushing water, vinegar, or commercial douching products into the vagina washes away the protective lactobacilli and disrupts the acidic pH they maintain. This creates an opening for anaerobic bacteria to move in. The vagina is self-cleaning, and internal rinsing does more harm than good. Washing the external vulva with mild soap and water is all that’s needed.
Smoking and Vaginal Health
Cigarette smoking has a surprisingly strong link to BV. A chemical byproduct of tobacco smoke called BPDE shows up in vaginal secretions of smokers, where it triggers the destruction of lactobacilli by activating viruses that naturally live dormant inside those bacteria. Smoking also has anti-estrogenic effects, and estrogen plays a role in maintaining a healthy vaginal lining.
The numbers are striking. In one study, smokers were seven times more likely to have the type of vaginal bacterial community associated with BV compared to a lactobacillus-dominated one. Smokers consistently have lower proportions of protective lactobacilli in their vaginal flora.
Copper IUDs and BV Risk
If you use an intrauterine device for birth control, the type matters. A 2023 review of available research found that copper IUD users may face a higher risk of BV, particularly during the first six months after insertion. Hormonal IUDs, by contrast, don’t appear to increase BV risk based on current evidence.
The effect seems partly dependent on where your vaginal flora starts. Women who already had an intermediate bacterial balance before IUD insertion (not fully dominated by lactobacilli, but not yet BV) were at greater risk of tipping into BV with IUD use.
Other Contributing Factors
Several additional factors can shift the vaginal balance toward BV:
- Antibiotics for other infections: Broad-spectrum antibiotics can kill off lactobacilli along with whatever infection they’re targeting, leaving the vagina vulnerable to anaerobic overgrowth.
- Hormonal changes: Fluctuations during menstruation, pregnancy, or menopause alter the vaginal environment. Menstrual blood temporarily raises vaginal pH, which is why some people notice BV symptoms around their period.
- Scented products: Fragranced soaps, bubble baths, and vaginal deodorants can irritate vaginal tissue and disturb the microbial balance.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. The biofilm that anaerobic bacteria build on the vaginal walls is central to this problem. Even after antibiotic treatment kills most of the bacteria, the biofilm structure can persist, allowing the community to rebuild. When other BV-associated species are present alongside Gardnerella, they can increase the biofilm’s resistance to antibiotics and immune defenses.
Re-exposure through sexual contact also plays a role. If a partner harbors BV-associated bacteria, reinfection can happen even after successful treatment. Some research suggests that treating partners simultaneously may help break this cycle, though this approach isn’t yet standard practice.
How BV Is Identified
BV is typically diagnosed when at least three of four signs are present: thin, milky discharge that coats the vaginal walls; clue cells visible under a microscope; vaginal pH above 4.5; and a fishy odor from the discharge. You might notice the odor is stronger after sex or during your period, since both raise vaginal pH and release the compounds responsible for the smell.
Not everyone with BV has obvious symptoms. About half of cases produce no noticeable signs, which means the condition can persist without your awareness. If you do notice unusual discharge, odor, or irritation, testing is straightforward and can be done during a routine exam.
How Treatment Works
BV is treated with antibiotics, typically taken either orally or applied as a vaginal gel or cream. A standard course runs about five to seven days. Symptoms usually improve within a few days of starting treatment, though finishing the full course matters for reducing the chance of recurrence.
Because of the biofilm problem, recurrence rates remain high. Roughly half of people treated for BV experience a return of symptoms within 12 months. Some providers recommend extended or periodic treatment for people with frequent recurrences. Probiotics and other strategies to rebuild lactobacillus populations are an area of active interest, but evidence for their effectiveness is still mixed.