Bacterial vaginosis (BV) happens when the normally acidic vaginal environment shifts toward a more alkaline state, allowing certain bacteria to overgrow and outnumber the protective bacteria that keep things in balance. It affects roughly 23 to 29% of women of reproductive age worldwide, making it the most common vaginal condition in that group. Despite how common it is, BV is widely misunderstood. It is not a sexually transmitted infection, and it is not caused by poor hygiene.
What Keeps the Vagina Healthy
A healthy vagina maintains a pH between 3.8 and 4.5, which is mildly acidic. That acidity comes from beneficial bacteria, primarily species called Lactobacillus crispatus, Lactobacillus jensenii, Lactobacillus gasseri, and Lactobacillus iners. These bacteria feed on glycogen, a sugar stored in vaginal tissue, and produce hydrogen peroxide and lactic acid as byproducts. That acidic environment makes it difficult for harmful bacteria to gain a foothold.
Estrogen plays a central role in keeping this system running. It stimulates vaginal cells to grow and store more glycogen, which feeds the protective bacteria. Progesterone helps break down those cells so the glycogen gets released. Together, these hormones sustain the conditions that keep the vaginal microbiome stable throughout the reproductive years.
The Shift That Causes BV
BV develops when something disrupts this balance and protective bacteria lose their dominance. As their numbers drop, the vaginal pH rises above 4.5. That less acidic environment allows anaerobic bacteria (the kind that thrive without oxygen) to multiply rapidly. These bacteria form a dense layer called a biofilm on the vaginal walls, which makes them harder for the body to clear on its own.
This isn’t a matter of “catching” a single germ. BV involves a community-level shift where many types of bacteria that were always present in small numbers suddenly proliferate. The result is a change in discharge, odor, and vaginal chemistry rather than a classic infection caused by one invading organism.
Common Triggers and Risk Factors
Douching
Douching is one of the strongest and most well-documented risk factors. It physically flushes out the hydrogen peroxide-producing bacteria that protect the vagina and disrupts the chemical environment they depend on. With that protective system weakened, anaerobic and other opportunistic bacteria overgrow. The irony is that many people douche to feel “cleaner,” but the practice makes BV more likely, not less.
Sexual Activity
Sex does not cause BV directly, but it creates conditions that can trigger it. Semen has a pH between 7.2 and 7.8, which is significantly more alkaline than the vagina. Exposure to semen temporarily raises vaginal pH, and if this happens frequently, it can destabilize the microbial balance. New sexual partners, multiple partners, and sex without condoms all correlate with higher BV rates, likely because each introduces new bacteria or repeated pH disruption. BV also occurs in women who have sex with women, suggesting that shared vaginal bacteria between partners plays a role.
Hormonal Changes
Because estrogen and progesterone drive the glycogen cycle that feeds protective bacteria, any hormonal shift can change the vaginal environment. Menstruation is a monthly disruption: blood has a near-neutral pH and temporarily raises vaginal alkalinity. Some people notice BV symptoms tend to appear right after their period for this reason. Hormonal contraceptives, pregnancy, perimenopause, and breastfeeding all alter hormone levels in ways that can shift the microbial balance in either direction.
Antibiotics
Antibiotics taken for other infections can kill off vaginal Lactobacillus along with whatever bacteria they were prescribed for. This collateral damage opens space for BV-associated bacteria to expand.
Smoking
Cigarette smoking is consistently linked to higher BV rates. The mechanism is not fully clear, but compounds from tobacco concentrate in vaginal secretions and appear to suppress Lactobacillus growth.
What BV Feels and Looks Like
About half of people with BV have no symptoms at all. When symptoms do appear, the most recognizable is a thin, grayish-white discharge with a strong fishy smell. The odor tends to be most noticeable after sex or during menstruation, both of which raise vaginal pH and release the volatile compounds that produce the smell. The discharge typically has a milklike consistency and coats the vaginal walls evenly rather than appearing in clumps.
BV does not usually cause significant itching, redness, or swelling. If those symptoms are prominent, a yeast infection or another condition is more likely. Mild irritation or burning during urination can occur with BV, but intense discomfort points elsewhere.
How BV Is Identified
Clinicians typically use a set of four criteria. A BV diagnosis requires at least three of the following: the characteristic thin, homogeneous discharge; vaginal pH above 4.5; a fishy odor when a chemical solution is added to a discharge sample; and the presence of “clue cells” under a microscope, which are vaginal cells visibly coated in bacteria. A lab-based scoring method that examines the ratio of bacterial types in a stained sample is considered the gold standard, though the clinical criteria are used more often in practice.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. More than half of people treated for it experience another episode within 12 months. The biofilm that BV-associated bacteria form on vaginal walls is a major reason. Antibiotics can reduce the bacteria enough to resolve symptoms, but the biofilm often survives at low levels and re-expands once treatment stops. If the underlying triggers, like douching, smoking, or repeated pH disruption from unprotected sex, continue, the environment keeps favoring the wrong bacteria.
Sexual partners can also harbor BV-associated bacteria, which may reintroduce them. This is an active area of investigation, and some clinicians now consider treating partners as well, though guidelines vary.
Health Risks of Untreated BV
BV is not just a nuisance. The shift in vaginal bacteria weakens the mucosal barrier that normally helps block pathogens. People with BV are more susceptible to contracting HIV, chlamydia, and gonorrhea. BV also increases the risk of transmitting HIV to a partner. Chlamydia and gonorrhea infections, in turn, can progress to pelvic inflammatory disease, which can cause chronic pain and fertility problems.
During pregnancy, untreated BV raises the likelihood of preterm birth and low birth weight. Pregnant people experiencing unusual discharge or odor benefit from early screening and treatment to reduce these risks.
Reducing Your Risk
The vagina is self-cleaning, and the single most effective thing you can do is stop interfering with that process. Avoid douching entirely. Wash the external vulva with warm water or a mild, unscented soap, and leave the internal vaginal canal alone. Scented products, including washes, sprays, and scented tampons or pads, can shift pH and irritate tissue.
Using condoms reduces repeated exposure to the alkaline pH of semen and limits the exchange of bacteria between partners. Choosing cotton underwear and avoiding prolonged time in wet clothing (like swimsuits) helps keep the area dry, since moisture encourages bacterial overgrowth. Quitting smoking removes a known chemical stressor on vaginal bacteria.
Probiotic supplements and foods containing Lactobacillus are widely marketed for vaginal health. Some small studies suggest benefit, particularly with strains matching the species that naturally dominate the vagina, but evidence is not strong enough to call probiotics a reliable prevention strategy on their own. They are most likely to help as a complement to other changes rather than a standalone fix.