Bacterial vaginosis (BV) happens when the protective bacteria that normally dominate the vagina decline sharply, allowing other bacteria to multiply and take over. It affects roughly 23 to 29% of women of reproductive age worldwide, making it the most common vaginal condition. The underlying cause isn’t a single germ or a lack of hygiene. It’s a shift in the entire bacterial ecosystem inside the vagina, and a surprisingly wide range of everyday factors can set that shift in motion.
How the Vaginal Microbiome Normally Works
A healthy vagina is home to large numbers of bacteria called Lactobacillus. These bacteria feed on a sugar called glycogen that the vaginal lining produces, and they convert it into lactic acid. That lactic acid keeps the vaginal pH low (acidic, usually between 3.8 and 4.5), which creates an environment hostile to most harmful bacteria. Lactobacillus species also produce other antimicrobial compounds that further suppress unwanted microbes.
Estrogen is the hormone that drives this whole system. It stimulates the cells lining the vaginal walls to deposit glycogen, which gives Lactobacillus the fuel it needs to thrive. This is why BV risk changes across a woman’s life. Before puberty and after menopause, when estrogen levels are low, there’s less glycogen, fewer Lactobacillus, and a higher vaginal pH. During the menstrual cycle, Lactobacillus populations can dip when estrogen temporarily drops, creating brief windows of vulnerability.
What Happens During BV
BV is not an infection caused by one invading organism. It’s a community-wide collapse. Lactobacillus numbers plummet, the vagina becomes less acidic, and a mix of anaerobic bacteria (species that thrive without oxygen) rushes in to fill the gap. The most common of these is Gardnerella vaginalis, but dozens of other species can be involved.
What makes BV particularly stubborn is what these bacteria do once they establish themselves. Gardnerella species are especially good at forming biofilms: dense, sticky mats of bacteria coated in a protective layer. Other BV-associated bacteria attach to the Gardnerella scaffolding and join the biofilm. Once embedded, these bacteria are shielded from both the immune system and antibiotics. This biofilm is a major reason BV comes back so often. Within 6 to 12 months of finishing antibiotic treatment, 50 to 80% of women experience a recurrence.
Common Triggers That Disrupt the Balance
Douching and Vaginal Washing
Douching is one of the strongest and most well-documented risk factors. Washing the inside of the vagina with water, soap, or commercial products disrupts the microbial community, causes inflammation, and creates an opening for harmful bacteria to colonize. Women who douched within the previous seven days had roughly twice the odds of developing BV compared to women who didn’t. In one study, women who had douched in the past six months had seven times the odds of reporting BV. Regular douching was also associated with significantly higher detection rates of Gardnerella vaginalis and other BV-linked bacteria. The vagina is self-cleaning, and internal washing works against the very system designed to keep it healthy.
Sexual Activity and Semen Exposure
BV is not a sexually transmitted infection, but sexual activity is closely tied to its development. Semen has a pH between 7.2 and 7.8, which is substantially more alkaline than the vagina’s normal acidic range. When semen enters the vagina, it temporarily raises the pH, weakening the acidic environment that Lactobacillus depends on. Repeated exposure, especially without condoms, gives anaerobic bacteria more opportunities to gain a foothold. New sexual partners or multiple partners further increase risk, likely because each partner introduces a different set of microbes that can challenge the existing community.
Menstrual Blood and Hormonal Shifts
Blood is also more alkaline than the vagina, so menstruation temporarily raises vaginal pH each month. This is why many women notice BV symptoms right after their period ends. Hormonal contraceptives, pregnancy, and perimenopause all alter estrogen levels, which in turn affect glycogen production and Lactobacillus populations. Any significant hormonal change can tip the balance.
Smoking and Stress
Cigarette smoking is consistently associated with higher BV prevalence. The exact mechanism isn’t fully understood, but nicotine and its byproducts have been detected in cervical mucus and may directly harm Lactobacillus. Psychological stress also plays a role. A higher number of stressful life events has been linked to increased BV prevalence, possibly through stress hormones that affect immune function and vaginal inflammation.
Why Some Women Are More Susceptible
BV prevalence varies dramatically across populations. In one large U.S. study, 45% of Black American women tested positive for BV compared to 17% of white American women. This disparity is not fully explained by behavioral differences. Among Black women, low income was strongly associated with elevated BV prevalence, but this same effect was not seen among white women. Stressful life events predicted BV in both groups, while smoking and relationship status were significant predictors only among Black women.
Researchers believe these disparities reflect an intersection of biological, socioeconomic, and psychosocial factors rather than any single cause. Chronic stress related to racial discrimination, differences in access to healthcare, neighborhood-level economic conditions, and possibly genetic variations in immune response all appear to contribute. BV is also linked to preterm birth, and the connection between BV and preterm delivery is stronger among Black women, making these disparities a significant public health concern.
Why BV Keeps Coming Back
The recurrence rate of BV is notoriously high. Standard antibiotic treatment clears the symptoms effectively, but it often fails to fully eliminate the Gardnerella biofilm clinging to the vaginal lining. These biofilms act like a reservoir. Once antibiotic levels drop, the surviving bacteria in the biofilm repopulate the vagina and restart the cycle. Sexual partners can also harbor BV-associated bacteria and reintroduce them.
This is why many women feel trapped in a pattern of treatment and relapse. Addressing the underlying triggers, such as stopping douching, using condoms to reduce semen exposure, and managing stress, can help reduce recurrence alongside medical treatment. Some clinicians now use longer or combination antibiotic courses specifically aimed at disrupting biofilms, though no single approach has solved the recurrence problem completely.
Recognizing BV Symptoms
About half of women with BV have no symptoms at all, which is part of why it’s so common. When symptoms do appear, the hallmark is a thin, grayish-white discharge with a noticeable fishy odor, particularly after sex or during menstruation. The odor comes from compounds released by the anaerobic bacteria. Unlike yeast infections, BV typically does not cause significant itching, redness, or thick discharge. If you notice a change in vaginal odor or discharge consistency, that pattern points more toward BV than other common conditions.
Diagnosis usually involves a vaginal swab. Clinicians look for characteristic “clue cells” (vaginal cells coated in bacteria), elevated pH above 4.5, and the fishy odor. A lab scoring method grades the ratio of Lactobacillus to other bacterial types under a microscope on a scale of 0 to 10, with a score of 7 or higher confirming BV.