Bacterial vaginosis is not classified as a sexually transmitted infection, but the relationship between BV and sex is more complicated than a simple “no.” BV is a vaginal condition caused by an imbalance in naturally occurring bacteria, not by a specific pathogen passed between partners the way chlamydia or gonorrhea would be. Yet it rarely occurs in people who have never had sex, and sexual activity is the strongest known trigger for developing it.
That gray area is exactly why this question comes up so often. Understanding what BV actually is, how sex contributes to it, and why it behaves differently from a classic STI can help you make sense of a diagnosis and reduce your chances of dealing with it again.
Why BV Isn’t Classified as an STI
A sexually transmitted infection involves a specific organism, like a bacterium or virus, that passes from one person to another during sex. BV doesn’t work that way. It’s a shift in the balance of bacteria already living in the vagina. Normally, protective bacteria called lactobacilli dominate the vaginal environment, keeping the pH acidic (below 4.5) and suppressing the growth of other microbes. In BV, lactobacilli decline and a mix of other bacteria, primarily one called Gardnerella vaginalis, proliferates and forms a sticky layer on the vaginal walls that allows additional species to thrive.
There’s no single “BV germ” that gets transmitted. The bacteria involved are naturally present in small numbers in most vaginas. What changes is their proportion, not whether they’re there at all. This is fundamentally different from catching an STI, where an outside pathogen enters the body through sexual contact.
How Sex Triggers the Imbalance
Even though BV isn’t an STI, sexual activity is deeply involved. Semen is alkaline, with a pH well above the 4.5 threshold that lactobacilli need to thrive. After unprotected intercourse, studies show a significant drop in lactobacillus levels and a corresponding rise in BV-associated bacteria like Gardnerella and Prevotella. The alkaline environment essentially gives these bacteria a temporary advantage. One study found that women with evidence of recent semen exposure had roughly 2.6 times the risk of having BV compared to those without exposure.
New sexual partners also increase risk. Each new partner introduces a different microbial community, and adjusting to those unfamiliar bacteria can destabilize the vaginal ecosystem. This pattern, where BV clusters around new partnerships and unprotected sex, is one reason it gets confused with STIs so frequently.
BV Between Female Partners
Some of the strongest evidence for sexual transmission of BV-associated bacteria comes from studies of women who have sex with women. The CDC notes that BV is even more common among women with female partners than in the general population. Having a partner who currently has BV, having a new partner, and sharing of vaginal or oral bacteria during sex are all associated with developing the condition. Researchers have found that BV-linked bacteria can live in oral and rectal reservoirs, meaning they can potentially be exchanged between partners through multiple types of sexual contact.
This complicates the “BV isn’t an STI” framing, because in female partnerships, the specific bacteria do appear to pass between people. The CDC states that epidemiologic data “strongly demonstrate” sexual transmission of BV among women with female partners, even though the condition still isn’t classified as a traditional STI.
Why Recurrence Is So Common
One of the most frustrating aspects of BV is how often it comes back. Up to 66% of women experience a recurrence within a year of their initial treatment. For a long time, the medical consensus was that treating a woman’s male sexual partner didn’t help prevent recurrence. That position has recently shifted.
In October 2025, the American College of Obstetricians and Gynecologists recommended for the first time that male sexual partners receive concurrent treatment when a woman has recurrent BV. The updated guidance cites growing evidence that sexual activity plays a direct role in both the initial occurrence and recurrence of BV, and that bacteria harbored by male partners may be reintroducing the imbalance after treatment. This is a significant change from prior guidelines and suggests the line between BV and STIs may be blurrier than previously thought.
Symptoms and Diagnosis
Many people with BV have no symptoms at all. When symptoms do appear, the most common is a thin, grayish-white discharge with a noticeable fishy odor, often stronger after sex. You might also notice mild irritation, though intense itching or burning is more typical of a yeast infection than BV.
Doctors diagnose BV using a set of clinical signs. At least three of the following four need to be present: the characteristic thin discharge, a vaginal pH above 4.5, a fishy smell when a chemical solution is applied to a sample, and the presence of “clue cells” (vaginal cells coated in bacteria) under a microscope. Because BV symptoms can overlap with STIs and yeast infections, getting an accurate diagnosis matters before starting treatment.
How BV Is Treated
BV is treated with prescription antibiotics, either taken orally for about seven days or applied as a vaginal gel for five days. Treatment typically clears the infection within a week or two. Some people are prescribed shorter courses of a different antibiotic as an alternative.
Because recurrence rates are so high, some women end up cycling through multiple rounds of treatment. Using condoms consistently, which prevents semen from raising vaginal pH, is one of the few behavioral changes shown to reduce recurrence risk. If you’re in a long-term partnership and keep getting BV, the new recommendation to treat your male partner is worth discussing with your provider.
BV Raises Your Risk for Actual STIs
One of the most important reasons to take BV seriously is that it makes you more vulnerable to sexually transmitted infections, including HIV. The protective lactobacilli that decline during BV normally produce hydrogen peroxide, which helps kill incoming pathogens. Without that defense, and with a higher vaginal pH activating more immune cells that HIV can target, the vaginal lining becomes easier for infections to penetrate.
A meta-analysis of published studies found that BV increases the risk of acquiring HIV by approximately 60%. In practical terms, for a population of women with a 2% baseline risk of HIV, having BV would raise that to about 3.2%, translating to roughly one additional HIV case for every 80 to 90 women with BV. The condition also increases susceptibility to chlamydia and gonorrhea through similar mechanisms.
Risks During Pregnancy
BV during pregnancy is linked to a significantly higher chance of preterm birth. A large meta-analysis found that pregnant women with BV had roughly 1.4 to 1.8 times the risk of delivering early compared to those without the condition. Preterm birth carries its own cascade of health risks for the baby, making BV screening and treatment during pregnancy particularly important.