Bacterial vaginosis (BV) happens when the balance of bacteria inside the vagina shifts, allowing harmful bacteria to outnumber the beneficial ones that normally keep the environment healthy. It is the most common vaginal infection in women of reproductive age, and despite decades of research, scientists still don’t fully understand why some women develop it and others don’t. What is clear is that several specific triggers can tip that bacterial balance in the wrong direction.
How the Vaginal Ecosystem Normally Works
A healthy vagina is home to large colonies of beneficial bacteria, primarily Lactobacillus species. These bacteria ferment sugars and produce lactic acid, which keeps vaginal pH at around 4, an acidic level that discourages most harmful organisms from taking hold. Lactobacillus species also generate hydrogen peroxide, a compound that directly damages the DNA of invading bacteria. Together, these defenses create an environment where potential pathogens simply can’t compete.
When something disrupts this system, Lactobacillus populations drop and the vagina becomes less acidic. A vaginal pH above 4.5 signals that the protective acid barrier has weakened. Anaerobic bacteria, organisms that thrive in low-oxygen, less-acidic conditions, seize the opportunity to multiply rapidly. The species most commonly involved in BV include Gardnerella vaginalis, Atopobium vaginae, and Prevotella bivia. Their overgrowth is what produces the hallmark symptoms: a thin, milklike discharge, a fishy odor, and sometimes mild irritation.
Sexual Activity and New Partners
BV is not classified as a sexually transmitted infection, but sexual activity is the single strongest association. The CDC notes that BV most often occurs in people who are sexually active and rarely affects those who have never had sex. Having new or multiple sexual partners increases risk, though researchers still don’t fully understand the mechanism. One theory is that exposure to a new partner’s genital bacteria introduces unfamiliar organisms that destabilize the existing Lactobacillus community. Not using condoms also raises the likelihood of BV, possibly because semen is alkaline and temporarily raises vaginal pH.
The relationship between sex and BV is complicated further by the fact that BV can and does occur without any sexual contact at all. This is part of why it’s not labeled an STI, even though sexual behavior is a major risk factor.
Douching and Hygiene Products
Douching, the practice of flushing water or cleansing solutions into the vagina, is one of the most well-documented triggers for BV. The process physically washes away Lactobacillus colonies, disrupts their protective acid environment, and can cause low-grade inflammation that makes it easier for harmful bacteria to colonize the area. Scented soaps, vaginal deodorants, and other products applied internally can produce a similar effect by altering the chemical environment that beneficial bacteria depend on.
The vagina is self-cleaning. External washing of the vulva with mild, unscented soap is sufficient. Anything introduced inside the vaginal canal risks disturbing the microbial balance that keeps BV at bay.
Other Risk Factors
Beyond sex and douching, several additional factors can shift the vaginal ecosystem toward BV:
- Antibiotics for other infections. Broad-spectrum antibiotics don’t discriminate between good and bad bacteria. A course of antibiotics for a urinary tract infection or sinus infection can inadvertently reduce Lactobacillus populations in the vagina.
- Hormonal changes. Fluctuations in estrogen, whether from your menstrual cycle, hormonal contraceptives, or menopause, affect how much glycogen (the sugar Lactobacillus feeds on) is available in vaginal tissue. Lower estrogen generally means less fuel for protective bacteria.
- Smoking. Tobacco use is consistently linked to higher BV rates. The exact pathway isn’t fully mapped, but compounds in cigarette smoke appear to reduce Lactobacillus colonization.
- IUDs. Some studies suggest intrauterine devices may modestly increase BV risk, possibly by providing a surface where biofilms of harmful bacteria can form.
Why BV Keeps Coming Back
One of the most frustrating aspects of BV is its recurrence rate. Between 50 and 80 percent of women experience BV again within 6 to 12 months of completing antibiotic treatment. Antibiotics effectively kill the overgrown anaerobic bacteria, but they don’t necessarily restore a robust Lactobacillus population. If the protective community doesn’t fully re-establish itself before exposure to a new trigger, the cycle starts over.
Biofilms also play a role. Gardnerella vaginalis and other BV-associated organisms can form a sticky, antibiotic-resistant layer on the vaginal wall. Standard treatment may suppress these biofilms without completely eliminating them, allowing bacteria to rebound once antibiotics are stopped. This is an active area of investigation, and it explains why many women feel stuck in a pattern of treatment, relief, and relapse.
How BV Is Recognized
Many women with BV have no symptoms at all. When symptoms do appear, the most common are a grayish-white, thin discharge with a noticeable fishy smell, particularly after sex. There’s usually no significant redness or swelling, which helps distinguish BV from yeast infections.
Clinicians typically diagnose BV using a set of criteria that includes checking for the characteristic discharge, measuring vaginal pH (above 4.5 suggests BV), looking for “clue cells” under a microscope (vaginal cells coated in bacteria), and testing whether the discharge produces a fishy odor when exposed to a chemical solution. Meeting at least three of these four markers confirms the diagnosis.
Health Risks of Untreated BV
BV often resolves on its own, but leaving it untreated carries real risks. The disrupted vaginal environment makes you more susceptible to sexually transmitted infections, including chlamydia, gonorrhea, and HIV, because the protective bacterial barrier is compromised. During pregnancy, BV is associated with preterm delivery, early miscarriage, low birth weight, and postpartum uterine infection. For anyone undergoing gynecologic procedures, untreated BV also raises the chance of post-surgical infection.
Even outside of these specific scenarios, persistent BV affects quality of life. The odor and discharge can cause significant anxiety and self-consciousness, and the high recurrence rate means many women deal with it repeatedly over months or years.