Bacterial vaginosis (BV) happens when the balance of bacteria in the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate and keep the environment acidic, which prevents harmful bacteria from thriving. When something disrupts that balance, other bacteria multiply and take over. The frustrating truth is that there’s rarely one single cause you can point to. BV typically results from a combination of factors, and understanding them can help you prevent it from coming back.
What Actually Changes Inside the Vagina
A healthy vagina is dominated by lactobacilli, which produce lactic acid and maintain a pH of around 3.8. That acidity is a defense system: it damages the outer membranes of harmful bacteria, creates osmotic stress that inhibits their growth, and boosts other immune defenses in the vaginal lining.
When lactobacilli lose their foothold, the pH rises and a diverse mix of anaerobic bacteria moves in. These include species like Gardnerella, Prevotella, and Mobiluncus. One key player, Gardnerella vaginalis, adheres strongly to vaginal cells and builds a sticky, protective structure called a biofilm. That biofilm acts like scaffolding: once it’s established, other harmful bacteria latch onto it and the community becomes self-reinforcing. This is a major reason BV can be so stubborn. The biofilm physically shields bacteria from antibiotics and from your immune system, and research shows it can persist on vaginal tissue even after a full course of standard antibiotic treatment.
Sex Is a Major Factor, but BV Isn’t an STI
BV is strongly linked to sexual activity, but it’s not a sexually transmitted infection in the traditional sense. You don’t “catch” it from a partner the way you catch chlamydia. Instead, sex changes the vaginal environment in ways that make BV more likely.
Semen is alkaline, with a pH much higher than the vagina’s acidic baseline. After unprotected sex, vaginal pH rises to about 5.5 within a few hours, and it can remain elevated at 5.2 even 10 to 14 hours later. That temporary alkaline shift directly promotes the growth of the anaerobic bacteria behind BV while suppressing the protective lactobacilli. Factors in semen also appear to directly inhibit lactobacilli and encourage pathogenic bacterial growth.
New sexual partners introduce unfamiliar bacteria. Having multiple partners or a new partner increases the likelihood that your vaginal microbiome will be disrupted. BV is also common among women who have sex with women, likely because bacteria transfer between partners through shared vaginal contact. Using condoms consistently reduces the risk because they prevent semen exposure and limit bacterial exchange.
Douching and Hygiene Products
Douching is one of the clearest, most avoidable risk factors. Women who douche at least once a month have significantly higher rates of BV, and those who douched within the past seven days have roughly double the risk compared to women who don’t douche at all. Douching washes away lactobacilli and directly promotes colonization by the bacteria that cause BV, including Gardnerella vaginalis.
It doesn’t matter why you douche. Women who douche for hygiene and women who douche to manage symptoms both show elevated risk. The vagina is self-cleaning, and introducing water, vinegar, or commercial douching products disrupts the microbial balance it maintains on its own. Scented soaps, sprays, and bubble baths applied to the vulva or vaginal area can cause similar disruption, though the evidence is strongest for douching specifically.
Other Common Triggers
Antibiotics taken for unrelated infections can wipe out lactobacilli along with the bacteria they’re targeting. If you developed BV after a course of antibiotics for something else, that’s a likely explanation. Hormonal changes, including those from certain birth control methods or the natural fluctuations of your menstrual cycle, can also shift the vaginal environment. Smoking is another established risk factor, though the exact mechanism isn’t fully understood.
Stress and immune suppression play a role too. Anything that weakens your body’s ability to support a healthy microbiome gives opportunistic bacteria an opening. IUDs, particularly in the first few months after insertion, have been associated with increased BV risk in some women.
Why BV Keeps Coming Back
If you’ve had BV before, you already know it has a frustrating tendency to return. The numbers bear this out: within 6 to 12 months of finishing antibiotic therapy, 50% to 80% of women experience a recurrence. That’s not a reflection of doing something wrong. It’s a consequence of BV’s biology.
The biofilms that Gardnerella builds on vaginal tissue are the core problem. Standard antibiotics kill free-floating bacteria effectively but struggle to penetrate the biofilm’s protective structure. Even after symptoms resolve and a test comes back normal, remnants of the biofilm can persist. When conditions shift again (a new partner, a period, a course of antibiotics), those remnants can rapidly reestablish the BV-associated bacterial community. Some of the bacteria living within these mixed biofilms also develop increased tolerance to antibiotics over time, making each recurrence potentially harder to treat.
Why It Matters Beyond Symptoms
BV is often treated as a nuisance, something that causes odor and discharge but nothing serious. That undersells the real risks. BV compromises multiple layers of your body’s defenses against sexually transmitted infections.
The protective mucus lining of the vagina becomes thinner and less viscous during BV, which allows viruses like HIV to move through it more easily. The bacteria involved in BV also produce toxins that damage vaginal tissue cells, creating gaps in the epithelial barrier that viruses can exploit to reach deeper tissue. On top of that, BV triggers inflammation that draws immune cells to the vaginal lining. Paradoxically, some of those immune cells are the very targets that HIV infects, so their increased presence near the surface actually raises transmission risk.
These same mechanisms, weakened mucus, damaged tissue, and heightened inflammation, likely apply to other infections including HPV and herpes. During pregnancy, BV is associated with preterm birth and low birth weight, which is why screening and treatment during pregnancy are taken seriously.
What You Can Do Differently
You can’t always prevent BV, but you can reduce the odds. Stop douching entirely. Use condoms with new or casual partners to limit both semen exposure and bacterial exchange. Choose unscented products for anything that contacts the vulva. If you notice that BV tends to follow your period, that’s the pH shift from menstrual blood creating a temporary window for anaerobic bacteria to gain ground.
Some women find that taking oral or vaginal probiotics containing specific lactobacillus strains helps restore the balance after treatment, though the evidence on this is mixed. What’s clear is that the goal is always the same: support the conditions that let lactobacilli thrive. That means an acidic, undisturbed vaginal environment with minimal exposure to alkaline substances.