What Causes Bacterial Vaginosis? A Clear Look at the Bacterial Imbalance Behind BV

You noticed a thin gray discharge and a faint fishy smell, maybe stronger after sex, and now you are wondering what you did wrong. The honest answer is reassuring: bacterial vaginosis is one of the most common vaginal conditions in people of reproductive age, and it usually is not about hygiene or doing anything wrong at all.

Bacterial vaginosis, or BV, happens when the normal balance of bacteria in your vagina gets disrupted. Here is what actually drives that imbalance, what raises your risk, and when it is worth talking to a clinician.

What causes bacterial vaginosis?

Bacterial vaginosis is caused by a shift in the community of bacteria that normally live in the vagina, a state called vaginal dysbiosis. In a healthy vagina, protective Lactobacillus species dominate. These bacteria produce hydrogen peroxide and lactic acid, which keep the environment slightly acidic and unfriendly to other microbes.

In BV, those protective lactobacilli drop and are replaced by high concentrations of anaerobic bacteria, the kinds that thrive without oxygen. The species most often involved include Gardnerella vaginalis, Prevotella, Mobiluncus, and Atopobium (A.) vaginae. When these anaerobes overgrow, they change the chemistry and smell of vaginal fluid, which is where the classic symptoms come from.

Importantly, researchers still do not know exactly why this imbalance starts in the first place. BV is not a single germ you catch like a cold. It is a change in an entire ecosystem, and several things can tip that ecosystem out of balance.

How common is BV, and who gets it?

BV is the most common cause of vaginal discharge and odor in women of reproductive age, so if you have it, you are far from alone. In a nationally representative U.S. survey (NHANES 2001-2004), an estimated 29.2% of women aged 14 to 49 had BV, which works out to roughly 21 million women.

Prevalence also varied notably by race and ethnicity in that analysis, ranging from about a quarter to about half of women depending on the group:

• 51.4% among non-Hispanic Black women
• 31.9% among Mexican-American women
• 23.2% among non-Hispanic White women

What raises your risk of bacterial vaginosis?

While the root trigger of the imbalance is unknown, public-health and medical sources agree on several factors that disturb normal vaginal flora and raise the odds of BV. BV occurs most often in people who are sexually active, though it is not classified as a simple sexually transmitted infection in the traditional sense.

Established and frequently cited risk factors include:

• Douching, which rinses out protective bacteria and disrupts the natural balance
• Having new or multiple sex partners
• Not using condoms
• Menstruation and sex, which Mayo Clinic lists among common triggers of the shift

Is BV sexually transmitted? The Gardnerella theory

This is one of the most debated questions in BV research. BV is strongly linked to sexual activity, but scientists have not labeled it a classic STI because it does not behave like one.

One leading model of how BV gets started proposes that it is initiated by sexually transmitted Gardnerella vaginalis. In this model, Gardnerella adheres to the vaginal lining and forms a biofilm, a protective bacterial layer, that lets other anaerobes overgrow and crowd out the protective lactobacilli. An alternative view holds that several bacterial species acting together are needed to produce the full picture of BV. Both ideas point to the same practical takeaway: the balance of bacteria, not one single culprit, is what matters.

What does BV feel like, and why do many people miss it?

When BV does cause symptoms, the hallmark is a thin grayish-white discharge with a foul, fishy odor that is often more noticeable after sex. There is usually little of the intense itching or thick, cottage-cheese discharge people associate with a yeast infection, which is one way the two differ.

Here is the part that surprises people: most cases are silent. Up to roughly 84% of women with BV report no symptoms at all. That is one reason BV can come and go unnoticed, and why a clinician's evaluation, rather than guessing, is the reliable way to confirm it.

When should you see a doctor about BV?

BV is treatable, and a clinician can confirm it and prescribe the right treatment rather than leaving you to guess. It is worth getting evaluated if you have new or persistent vaginal discharge, an unusual or fishy odor, or symptoms that keep coming back.

Getting BV checked matters beyond comfort. When left untreated, BV is associated with a higher risk of acquiring HIV-1 and other sexually transmitted infections, pelvic inflammatory disease, and adverse pregnancy outcomes such as preterm labor and low-birth-weight delivery. If you are pregnant or planning to be, or if you have pelvic pain, fever, or symptoms that do not resolve, see a clinician promptly. A personalized assessment, whether in person or through a clinician-overseen service, is the safest path to the right treatment.