BV, or bacterial vaginosis, is the most common vaginal infection in women of reproductive age. It happens when the balance of bacteria inside the vagina shifts: the protective bacteria (lactobacilli) decline, and a mix of other bacteria overgrow in their place. Global estimates put the prevalence between 23% and 29% of reproductive-age women, making it far more common than most people realize.
BV is not a sexually transmitted infection, though sexual activity can increase the risk. It’s driven by a disruption in the vaginal microbiome, not by a single invading organism, which is why it behaves differently from infections like chlamydia or gonorrhea.
What Happens Inside the Vagina
A healthy vaginal environment is dominated by lactobacilli, a group of bacteria that produce lactic acid and keep the vaginal pH low (acidic). This acidity acts as a first line of defense against harmful organisms. When something disrupts that environment, lactobacilli lose their foothold and a diverse mix of bacteria fills the gap. The vagina shifts from a low-diversity, acidic state to a high-diversity state where organisms that normally exist in small numbers begin to flourish.
This shift also triggers changes in local immune function. The natural protective barrier weakens, which can make the vagina more vulnerable to further microbial changes and to sexually transmitted infections. It’s a self-reinforcing cycle: fewer protective bacteria means a less hostile environment for harmful ones, which further crowds out the remaining lactobacilli.
Symptoms to Recognize
The hallmark symptom is a thin, grayish discharge with a fishy odor. The discharge tends to be homogeneous (smooth and even, almost milklike) rather than clumpy, and it coats the vaginal walls. You may notice small bubbles in it. The odor is often most noticeable after sex, because semen is alkaline and can release the compounds responsible for the smell.
Some women with BV have no symptoms at all. When symptoms do appear, odor is typically the first thing noticed, sometimes before any visible change in discharge. BV does not usually cause significant itching or burning, which can help distinguish it from yeast infections, where itching is the dominant complaint, or from trichomoniasis, which often causes irritation and a greenish discharge.
What Increases Your Risk
The CDC identifies three main risk factors: douching, not using condoms, and having new or multiple sex partners. Douching is particularly damaging because it directly washes away lactobacilli and disrupts vaginal pH. Despite being marketed as a hygiene product, douching consistently increases the likelihood of BV rather than preventing it.
New sexual partners introduce unfamiliar bacteria into the vaginal environment, and unprotected sex compounds the effect. That said, BV also occurs in women who are not sexually active, which reinforces that it’s fundamentally about microbial balance rather than transmission of a specific pathogen. Other factors that may play a role include smoking, antibiotic use, and hormonal changes.
How BV Is Diagnosed
Diagnosis usually happens in a clinic using a set of criteria called the Amsel criteria. A clinician looks for at least three of four signs: the characteristic thin, homogeneous discharge; a vaginal pH above 4.5; a fishy smell when a chemical solution is applied to a sample of the discharge; and the presence of “clue cells” under a microscope (vaginal cells with bacteria visibly stuck to their surface).
A more precise lab method involves examining a vaginal sample under a microscope and scoring it on a 0 to 10 scale based on the types of bacteria visible. A score of 0 to 3 indicates a healthy, lactobacillus-dominant environment. A score of 4 to 6 suggests an intermediate state. A score of 7 to 10 confirms BV. This lab method is considered the reference standard, though many providers rely on the clinical criteria because results are available immediately.
Treatment and the Recurrence Problem
BV is treated with antibiotics, typically taken either orally or applied as a vaginal gel or cream. Treatment courses are short, usually lasting about a week, and most women see symptoms resolve within days of starting.
The bigger challenge is recurrence. Between 50% and 80% of women who complete antibiotic treatment experience BV again within 6 to 12 months. This high recurrence rate is one of the most frustrating aspects of BV. Antibiotics eliminate the overgrown bacteria, but they don’t necessarily restore a healthy lactobacillus-dominant environment. Once treatment ends, the same imbalance can re-establish itself.
For women dealing with repeated episodes, prevention strategies become critical. Avoiding douching, using condoms consistently, and limiting the introduction of new bacteria through sexual contact all reduce risk. Some women find that paying attention to what triggers their episodes (a new partner, a course of antibiotics for something else, a change in products used near the vagina) helps them anticipate and sometimes prevent recurrences.
Probiotics and Restoring Balance
Because BV is fundamentally a problem of missing lactobacilli, researchers have been studying whether probiotic supplements can help restore and maintain a healthy vaginal microbiome. One randomized, placebo-controlled trial tested an oral probiotic (a strain of Lacticaseibacillus rhamnosus) in women with BV. After 10 days of daily capsules, the probiotic group showed significant reductions in all BV-associated pathogens and a meaningful increase in lactobacilli. These improvements persisted 30 days after treatment ended. Women in the placebo group showed no significant changes.
The probiotic group also saw improvements across all four Amsel diagnostic criteria, and their Nugent scores dropped enough that none of the participants still scored in the BV range by the end of the study. While this is promising, probiotics are not yet a standard replacement for antibiotics. They may be most useful as a complement to antibiotic treatment or as a strategy for reducing recurrence.
Why BV Matters During Pregnancy
Untreated BV during pregnancy carries real risks. In one study, the preterm birth rate before 34 weeks was 22.7% among women with BV compared to 6.2% among those without it. Babies born to mothers with BV had significantly lower birth weights (a median of 2,450 grams versus 2,950 grams) and were more than twice as likely to need intensive care after delivery.
Rates of respiratory distress syndrome were also substantially higher: 33.3% in the BV group versus 9.0% in the comparison group. More than half of women with BV in the study showed signs of infection in the placenta tissue. These numbers make a strong case for screening and treating BV during pregnancy, even in the absence of bothersome symptoms.
BV and Susceptibility to Other Infections
The loss of protective lactobacilli doesn’t just cause BV symptoms. It also leaves the vaginal lining more vulnerable to sexually transmitted infections, including HIV, herpes, and chlamydia. The weakened immune environment and higher pH create conditions where these pathogens can establish themselves more easily. For women at risk of STI exposure, managing and preventing BV is one practical way to reduce that vulnerability.