Bacterial vaginosis (BV) develops when the natural balance of bacteria in the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment, keeping it acidic and protective. When those lactobacilli decline and are replaced by a mix of other bacteria, the result is BV. It’s the most common vaginal condition in women of reproductive age, yet the exact trigger for this bacterial shift isn’t fully understood.
How Vaginal Bacteria Normally Stay in Balance
A healthy vagina is home to large numbers of lactobacilli, rod-shaped bacteria that produce hydrogen peroxide and lactic acid. These compounds keep the vaginal pH below 4.5, creating an environment that’s inhospitable to most harmful microorganisms. Two species in particular, L. crispatus and L. jensenii, are especially effective at producing hydrogen peroxide, which is toxic to many anaerobic bacteria and has even been shown in lab studies to inhibit certain viruses.
This acidic, lactobacillus-dominated environment is self-reinforcing. The beneficial bacteria form a thin layer along the vaginal lining that acts as a living barrier. As long as lactobacilli remain the dominant population, potentially harmful bacteria are kept at low, harmless levels.
What Happens When the Balance Breaks
BV begins when something disrupts this protective system, allowing anaerobic bacteria to multiply. As lactobacilli decline, the vagina becomes less acidic, and the pH rises above 4.5. This more alkaline environment favors the growth of organisms that were previously kept in check.
The bacterium most closely associated with BV is Gardnerella vaginalis, which forms a sticky biofilm on the vaginal lining. This biofilm is remarkably durable. Studies show it can survive in hydrogen peroxide, lactic acid, and even high concentrations of antibiotics, which helps explain why BV so often recurs after treatment. But BV is not a single-organism infection. It involves a whole community of bacteria working together, including Prevotella, Mobiluncus, Atopobium vaginae, and others. Researchers describe it as a “polymicrobic synergistic infection,” meaning no single bacterium is solely responsible.
Once established, this bacterial mix produces compounds that give BV its characteristic thin, milky discharge and fishy odor. The shift from a protective, acidic vagina to an alkaline, anaerobe-dominated one is the core of the condition.
The Role of Sexual Activity
BV is strongly linked to sexual activity, but it’s not classified as a sexually transmitted infection. The CDC notes that BV rarely affects women who have never had sex, and that new or multiple sexual partners increase the risk. Yet researchers still don’t fully understand the mechanism. Treating a sexual partner has not been shown to reduce recurrence, which is one reason BV doesn’t fit neatly into the STI category.
One leading theory involves virulent strains of Gardnerella vaginalis being introduced through sexual contact. After exposure, these strains can displace vaginal lactobacilli and begin forming the biofilm that anchors BV. This would explain the association with new partners without making BV a straightforward infection passed back and forth. Women who have sex with women also have elevated BV rates, suggesting that shared vaginal bacteria between partners plays a role.
Douching and Vaginal Products
Douching is one of the most clearly established risk factors. It directly disrupts the hydrogen peroxide and lactobacillus-based defense system of the vagina. Research published by the American Academy of Family Physicians found that douching at least once per month increased the risk of BV 1.4-fold. Women who had douched within one week of being studied had 2.1 times the risk.
The mechanism is straightforward: flushing the vaginal canal with water or chemical solutions physically removes lactobacilli and dilutes the acidic environment they maintain. This creates a window of vulnerability in which anaerobic bacteria can gain a foothold. Scented soaps, sprays, and other products used internally can have a similar effect, though the research on douching is the most robust.
Smoking as a Risk Factor
Cigarette smoking increases BV risk through a biological pathway most people wouldn’t expect. Smoking has an anti-estrogenic effect, which can thin the vaginal lining and reduce the glycogen that feeds lactobacilli. On top of that, a chemical byproduct of tobacco smoke called benzo[a]pyrene diol epoxide (BPDE) has been detected in the vaginal secretions of smokers. In lab studies, this compound triggers the activation of bacteriophages, viruses that specifically kill lactobacilli. So smoking essentially attacks the protective bacteria from two angles: starving them of resources and unleashing viruses that destroy them.
Other Factors That Raise Your Risk
Beyond the major contributors, several other factors are associated with BV:
- Not using condoms. The CDC lists unprotected sex as a risk factor. Semen is alkaline, with a pH around 7.2 to 8.0, which temporarily raises vaginal pH after intercourse. Condoms prevent this exposure.
- IUDs. Some evidence links intrauterine devices to a modestly higher BV rate, possibly because of changes to cervical mucus or the string acting as a surface for biofilm.
- Antibiotics. Broad-spectrum antibiotics taken for other infections can inadvertently kill lactobacilli, creating an opening for BV-associated bacteria.
- Hormonal changes. Estrogen supports the vaginal conditions that favor lactobacilli. Periods of low estrogen, such as during menstruation or perimenopause, can temporarily make the environment more vulnerable.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. Up to half of women treated for BV experience a recurrence within 12 months. The Gardnerella biofilm is a major reason. Because it adheres firmly to the vaginal lining and can withstand both the body’s natural defenses and antibiotic treatment, it’s difficult to fully eradicate. Even after symptoms resolve, remnants of the biofilm can persist and serve as a foundation for the bacterial community to rebuild.
Ongoing exposure to the same risk factors, whether that’s a sexual partner harboring Gardnerella, continued douching, or smoking, also keeps the cycle going. Addressing these modifiable risk factors is often as important as the treatment itself.
What BV Feels Like and How It’s Identified
Many women with BV have no symptoms at all. When symptoms do appear, the most common are a thin grayish-white discharge with a noticeable fishy smell, especially after sex. There’s typically no itching or burning, which helps distinguish BV from yeast infections.
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 when the discharge is exposed to a chemical solution, and the presence of “clue cells” under a microscope (vaginal cells coated with bacteria). A vaginal pH test alone can be a useful early indicator, since healthy vaginal pH sits below 4.5.
Complications When BV Goes Untreated
BV isn’t just a nuisance. The disrupted vaginal environment increases susceptibility to sexually transmitted infections, including HIV, chlamydia, and gonorrhea, because the protective lactobacillus barrier is compromised. During pregnancy, BV has been associated with preterm delivery, early miscarriage, low birth weight, and postpartum uterine infections. It can also increase the risk of infection after gynecological surgeries.
Because BV is often asymptomatic, many women carry it without knowing. This makes understanding the risk factors especially valuable: if you recognize the conditions that promote BV, you can take practical steps like avoiding douching, using condoms, and quitting smoking to keep your vaginal microbiome intact.