Bacterial vaginosis (BV) develops when the natural balance of bacteria in the vagina shifts. Normally, beneficial bacteria called lactobacilli keep the vaginal environment slightly acidic (below pH 4.5), which prevents other organisms from overgrowing. When something disrupts that balance, anaerobic bacteria multiply rapidly and take over, raising the pH and producing the telltale symptoms: a thin, milky discharge and a fishy odor. BV affects roughly 23 to 29% of women of reproductive age worldwide, making it the most common vaginal condition in this group.
What Happens Inside the Vagina
A healthy vagina is dominated by lactobacilli, which produce lactic acid and hydrogen peroxide. These create a hostile environment for harmful bacteria. In BV, that protective population shrinks and is replaced by a diverse mix of anaerobic bacteria. The most prominent is Gardnerella vaginalis, found on roughly 90% of the vaginal surfaces in women with BV. But Gardnerella alone doesn’t always cause problems. Many women carry it without developing symptoms.
What makes BV persistent and frustrating is biofilm. Gardnerella is especially good at attaching to vaginal cells, displacing the protective lactobacilli, and forming a sticky, layered community that other harmful bacteria join. This biofilm is more resistant to the natural acids and hydrogen peroxide that lactobacilli produce, which is a major reason BV keeps coming back after treatment. The biofilm essentially gives the problematic bacteria a shield.
Sexual Activity Is the Biggest Risk Factor
Many specialists now consider BV a sexually transmitted infection, though the relationship is more nuanced than with something like chlamydia. BV is strongly associated with having new or multiple sexual partners, and it’s more common in people whose partner also has BV. It can spread through shared sex toys, oral-genital contact, and fingers.
Women who have sex with women are at particular risk if a partner has BV, since the bacteria transfer directly between vaginal environments. Not using condoms also increases the likelihood. Semen is alkaline, which temporarily raises vaginal pH and may create a window for anaerobic bacteria to gain a foothold, though this mechanism is less well-established than the overall pattern linking sexual activity to BV.
That said, not every episode of BV requires a new sexual exposure. Once the bacterial balance has been disrupted and a biofilm is established, recurrences can happen on their own.
Douching and Other Non-Sexual Causes
Douching is one of the clearest non-sexual triggers. Women who douche at least once a month have a 1.4 times higher risk of developing BV. For women who douched within a week of being tested, the risk jumped to 2.1 times higher. The mechanism is straightforward: douching washes away the hydrogen peroxide and lactobacilli that protect the vagina, giving anaerobic bacteria room to expand.
Other factors that can shift the bacterial balance include:
- Antibiotics: Taking antibiotics for an unrelated infection can kill off lactobacilli along with the targeted bacteria, leaving the vagina vulnerable to overgrowth.
- Hormonal changes: Fluctuations during your menstrual cycle, particularly around your period when blood raises vaginal pH, can temporarily favor anaerobic bacteria.
- Scented products: Fragranced soaps, bubble baths, and vaginal deodorants can irritate the vaginal lining and disrupt its microbial balance.
You don’t need to be sexually active to get BV. While sexual activity is the most common pathway, anything that disrupts the vaginal environment can set the stage.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. Even after standard antibiotic treatment, a significant number of women see symptoms return. In one clinical trial, 45% of women treated with the standard antibiotic had a recurrence within 12 weeks. The Gardnerella biofilm is a major reason. Standard antibiotics can kill the free-floating bacteria but often fail to penetrate the biofilm completely, allowing the community to rebuild.
A promising approach involves restoring the protective bacteria directly. In the same trial, women who used a vaginal probiotic containing Lactobacillus crispatus twice weekly for 11 weeks after antibiotic treatment had a 30% recurrence rate at 12 weeks, compared to 45% in the placebo group. This suggests that replenishing the right strain of lactobacilli can help the vagina re-establish its defenses, though it’s not a guaranteed fix.
How BV Is Recognized
Doctors diagnose BV using a combination of clinical signs. A diagnosis typically requires at least three of the following four: a thin, homogeneous discharge that coats the vaginal walls; a vaginal pH above 4.5; a fishy odor (which becomes stronger when a chemical solution is added to a sample); and the presence of “clue cells,” which are vaginal cells visibly covered in bacteria under a microscope.
Many women with BV notice the odor first, particularly after sex. Some have no symptoms at all. About half of BV cases are asymptomatic, which means the bacterial shift can happen without you realizing it.
Health Risks of Untreated BV
BV isn’t just uncomfortable. It increases susceptibility to sexually transmitted infections, including HIV, because the loss of protective lactobacilli leaves the vaginal lining more vulnerable to pathogens. During pregnancy, BV has been linked to preterm delivery, early miscarriage, low birth weight, and postpartum uterine infections. In the US, about 10% of live births are preterm, and approximately two-thirds of infant deaths occur among babies born early.
Reducing Your Risk
You can’t control every factor, but several habits lower the odds of developing BV or having it return. Using condoms consistently reduces exposure to new bacteria and limits the pH-raising effect of semen. If you use sex toys, washing them after each use prevents bacterial transfer. Avoiding douching is one of the simplest and most effective steps, since it removes the very bacteria your vagina needs to stay healthy.
Hormonal contraception appears to offer some protection, likely because it stabilizes the hormonal fluctuations that can shift vaginal pH. Wearing breathable cotton underwear and avoiding scented products in the genital area also helps maintain a stable environment. The vagina is largely self-cleaning, and the less you interfere with that process, the better its bacterial ecosystem functions.